ZOMG! that's awesome! I've been using emacs for years, and never quite figured this out! SO helpful. no more saving things to a file called "foo" and dropping to a terminal to run wc, etc.:)
also...rectangles are the shit! every time I show friend some emacs, this is one of the first things i give 'em. they never cease to be amazed:)
For the interested, here is an online video of a presentation given by ken sembach, the HST project scientist, at a symposium earlier this year. In it, he describe the servicing mission (SM4) in detail, with a particular emphasis on the new instruments being installed (WFC3, COS) and those being repaired (STIS, ACS).
There some cool shots of the astronauts in the massive water tank that simulates zero-g, practicing removing all those screws with the specially designed screw-plate.
The inital "spark" is provided by gravity. When a star forms, it is from a slowly collapsing gas cloud. As the gas contracts under gravity, it heats up, eventually reaching a temperature for fusion to begin. The temperature is important, because the hydrogen nuclei (i.e. protons) normally repel each other...thus they need a fair bit of energy to overcome the repulsion, and get close enough to "stick"...i.e. fusion.
This is very similar to the ideal gas law, PV = nkT....i.e. as the pressure goes up, there must be a corresponding increase in temperature. A cool experiment to illustate this is to put your finger over the pointy end of a fat syringe, and then press really hard. The increased pressure will heat the gas. This is also way scuba tanks need to be filled in a bath of water...compressing the gas to put into the tank creates a fair amount of heat.
There is a little confusion about how the elements are created, and where HE1523 got all it's metals from...so here is a quick primer on the way things work.
The big bang forms hydrogen, dueterium, some helium, and a tiny amount of lithium. In fact, the theory of what should be formed (called Big Bang Nucleosynthesis), and what is observed, agree incredibly well.
Most stars just burn hydrogen into helium, fusing the two hydrogen atoms. More massive stars burn hotter, and so they can ignite helium burning, forming carbon, nitrogen, oxygen etc. The hotter the star gets, the heavier things can be fused, all the way up to iron. All of these processes *release* energy, if you can get it hot enough to start the reaction.
After iron, to make heavier elements you have to *put in* energy, so the way elements are formed is different. Instead of fusing two things together, you now just add a single neutron to the nucleus. This is a very different process (called neutron capture)...and can happen veeeery slowly (in stars) or very rapidly (in supernova explosions).
So, uranium and thorium are both elements which are made in the rapid process (r-process) -- they are only made in supernova explosions...because in a supernova, the neutron density is very high, so catching one is more likely.
Anyway...the point of all this is that, by observing uranium, we KNOW there had to have been at least one dying star going supernova, which made the uranium. Then that gas collapsed again later, to make anna's star.
So far, no-one has yet managed to find a first-generation star, but it's a big area of research at the moment, and is one of the things anna is trying hard to find. By looking at these very old stars, we get a good picture of how a supernova works, because we see the product of ONLY ONE of them. With young stars, there might have been hundreds, all polluting the gas at different times...and disentangling that is really tough.
As for the age of the universe, WMAP told us that very precisely -- 13.7Gyr (with an error of only ~0.1Gyr). The age we derived from HE1523 is much less precise...but nucleocosmochronometry (stellar age dating), is an incredibly tough thing to do, but it does offer independant confirmationg of the WMAP result.
That's correct. The star is metal-poor -- it's has an iron abundance (the standard measure of how much metals a star has) of [Fe/H] = -2.95. This is a lograthmic scale, and means that, on a scale where the sun is 0.0, HE1523 has about 1/1000th the amount of iron. The bracket notation means [Fe/H] = log10{N(Fe}/N(H)} - log10{N(Fe)/N(H)}_sun...i.e. the logarithmic difference of the number of atoms of Fe, compared to hydrogen, normalised to the solar ratio.
But the kicker is that HE1523 is very heavily r-process enhanced too...which means that it has a lot r-process, neutron-capture elements (think Uranium and thorium), compared to how much iron it has. HE1523 has [r/Fe] = 1.8....which means it has a 100 times more r-process heavy metals compared to iron, than does the sun.
BOTH of these factors are very important for this measurement, because you need to have very few metals, very high signal-to-noise data, very high resolution, and very strong r-process abundance, in order to be able to observe the uranium line. Anna needed 7.5hrs of VLT time to get a signal-to-noise ratio of about 350 or so...much higher than the S/N ~ 50-75 that we got from Magellan.
You can get a pdf of the paper here. Check out Fig 2, which shows the relevant part of the spectrum, with the Uranium line. See how it's right next to the booming Fe line...that's why we need a low iron abundance to do this work.
Your cost estimates are accurate, but the rest of your argument is total shit. Adaptive optics, WHEN it works (which is rarely, and with difficulty), can approach the angular resolution of HST in a VERY SMALL field of view. You cannot get 0.05 arcsec, diffraction limited images over a wide field of view, that is possible with HST.
"Designing a mirror to withstand a launch vehicle" is a problem that has been solved. And the only two current, viable telescope proposals for telescopes larger than 10m are the Thirty Metre Telescope (TMT) and the Giant Magellan Telescope (GMT). OWL is not a concept that is being taken very seriously...ESO certainly hasn't put its money where its mouth is.
Your final point, about not many lines in that part of the spectrum, belies a complete lack of understanding of what you are talking about. The UV (accessible with STIS, and the Cosmic Origins Spectrograph, which will fly on SM4 in late '08) are so full of lines that they overlap all over the place. See, for example, Morton (2003), ApJS, 149, 205, for a comprehensive list. At low redshift, lines of HI, OI, OVI, CIV, NV, CII, SiII, SII, FeII, NI...all are in the UV, in the STIS band. Furthermore, space is the ONLY place these wavelengths can be observed, because of the atmosphere is opaque to wavelengths shorter than about 3300 angstroms.
JWST will provide diffraction-limited images at 2 micron. It will have imaging and spectrographic capabilities in the near and mid-IR -- everything from 6000AA out to 27micron with the mid-IR imager and spectrograph (MIRI). StSci has a JWST primer online here (pdf link).
complete bullshit. This has nothing to do with dark matter, or the amount of visible mass. It merely tells you what percentage of the sun is comprised of oxygen. It is a very tiny amount. For every oxygen atom, there are about a 1000 hydrogen atoms and a hundred helium atoms.
The evidence for dark matter is based on other observations, like the way disk galaxy's rotate. In order to reproduce those observations, dark matter is required.
The estimate of the total amount of mass in various phases (e.g. stars, cold gas, hot gas, etc etc) in the universe has been done. You can read the paper here. Look at table 1. This is the contribution of all the different things to the total energy-density of the universe. What is amazing is the *tiny* fraction of the total energy-density that is made from baryons (visible, observable stuff). It's only about 4% or so. 23% is dark matter, and the rest is "dark energy".
How the solar oxygen abundance is derived
on
The Solar Oxygen Crisis
·
· Score: 4, Interesting
This is not a new issue in astrophysics, and has been floating since 2004. There are two basic ways to measure the abundances. One is by looking at hte oscillations in the sun, and using those to probe the solar interior. This is called "helioseismology", since it is very similar to the way seismologists figure out the structure and composition of hte earth, by observing seismic waves.
The other way is to take a spectrum of the sun (which is really just the solar photosphere -- the outer layers, or "atmosphere"). To interpret the spectra, one needs a model, which is used to derive the abundance (how much oxygen there is).
Now...until recently the models used for deriving abundances were simple 1-dimensional models, which made some assumptions (such as "local thermodynamic equilibrium") and include some fudge factors to account for the fact that you're solving a 3-d problem in 1-d.
The oxygen problem arises when you use accurate, 3-D models, which don't make the LTE assumption mentioned above -- called non-local thermodynamic equilibrium (NLTE). When one compares the abundances from the 3d NLTE models with what is expected from the helioseismology predictions, the discrepancy arises.
Others have posted the link to the full journal article on the pre-print server (here). The introduction of this paper is a pretty good summary of the problem, albeit intended for a scientific audience.
Yup. There was a paper a few years back entitled "terascale sneakernet", by jim gray and a couple of guys at MSFT research division on this. You can find it in the arxiv.
This concept has also been applied to such things as the Sloan Digital Sky Survey. Astronomers do tend to generate a lot of data with large surveys such as this.
actually, there is a grant flow-on from hubble operations, in that observing time on HST can translate directly into money (e.g. HST General Observer grants).
Further, HST has made many observations that are simply not possible from the ground, even with 8m-class telescope and adaptive optics (which are notoriously difficult to get working). E.g. observing in the UV is simply not possible from ground-based telescopes.
sorry, wrong again. HI, as defined and used by every astronomer on the planet, it neutral hydrogen. That's a H with a roman I next to it. HII is ionised hydrogen (H+ to chemists). H_2 is molecular hydrogen.
1) You definitely wouldn't see single stars. We'd see only the integrated light from a whole population of stars.
2) The numbers are already done for us. From the paper: 'We conclude that there is no optical counterpart to VIRGOHI21 down to a B-band surface-brightness limit of 27.5 B mag/arcsec^2. This is less than 1 solar luminosity pc^-2, giving a maximum luminosity in stars of less than 10^8 solar luminosities if a diameter of 16 kpc is assumed.'
3) M31 isn't far away at all. In fact, its the closest large galaxy to the MW. HST can resolve individual stars there, allowing us to measure the brightnesses and construct helpful "colour-magnitude diagrams" for instance.
4) No. Read the paper. They argue that the low surface density of gas prevents fragmentation of hte gas, and hence stars not forming.
The answer, as with most things is, it depends. As I said, the MW halo is a good example. the stars in the halo are almost purely pressure supported with almost zero net rotation (most measures are 10km/s). But this is decoupled from the rest of the galaxy. Clearly the more things collapse, the faster they will spin...hence the rotation of the disk (circular speed of about 220km/s at the solar radius). The MW bulge I'm not so sure about (could look it up...).
In general there will always be some net angular momentum...whether it is significant is really dependant on the formation history of the object.
errr...no, not really. Galaxies can also be "pressure supported"...in which case the stars have totally random motions. Each star has sufficient kinetic energy to prevent it falling in, but none of these velocities need be aligned. Thus all the stars are going in random directions, like atoms in a gas (hence the pressure reference).
In fact, stars in the halo of the milky way is pressure-supported with little or no bulk rotation. gas behaves quite differently, of course, since instabilities can form much more easily.
They observed the neutral hydrogen gas (HI), which emits radio waves at the well-known 21cm wavelength. This is not dark at all. From the rotation of the gas, we can work out, with a few assumptions, how big the gravitational potentional would be required in order that the gas is bound. This extra mass is assumed to be dark matter.
NOT TRUE!! The difference between the stars LSR and GSR velocities is similar from out point of view, because it's velocity vector points almost perpendicular to our motion around the galactic centre (see my other comment for more details about frames).
For observers 1/4 of the way around the galaxy in the disk at the same radius, it will be vastly different, since then the circular velocity aroudn the galactic centre is in the same dircection (roughly) as the star and so the difference between "LSR" and GSR frames will be significant.
All measurements of stellar velocities are not the stars true velocity - they are "radial" velocities. This is the geometric projection of the stars 3 dimensional velocity along the line of sight. Now, when you do this, everything is moving and many corrections must be made.
First, we correct for the rotation of the earth, and the movement of the earth around earth-moon barycentre. Then we correct for the earths movement around the sun. This is what is usually reported, and it is called the Heliocentric (i.e. centred on the sun) radial velocity.
Now the sun moves a little bit (about 20km/s) with respect to stars in our local neighbourhood, so we correct to this so-called "local standard of rest" or LSR.
From the LSR velocity, we can take out the movement of the sun aroudn the galactic centre, and put it in a galactocentric standard or rest, or GSR. This is done via the relation v_GSR = v_LSR + 220*sin(l)*cos(b), since the sun has a circular velocity around the galactic centre of about 220km/s. l and b here are the galactic latitude and longitude, with (l=0,b=0) the galactic centre and l increasing counter-clockwise.
So, what we really care about, is the GSR velocity, from which we can tell whether the star is bound or not (i.e. is v_GSR > escape velocity).
There is a link to the actual article here (which has been submitted).
BUT! Don't forget, we measure only 1 component of the velocity (towards/away from us) - we really need all 3 (side-to-side as well) which are called "proper motions". So it could in fact, have higher a higher velocity.
Sorry, but many of the points you make are correct, but you basic premise - that HST is obsolete, is wrong. For these reasons:
- Not all of "us" are asking the same questions - HST is incredibly versatile. I was waiting for COS to go up in the next service mission, since it was going to have a huge impact in my field, studying the possible accretion of gas into the milky way.
- Interferometry is only applicable in specific circumstances and is not a general replacement for diffraction-limited imaging at resolutions of about 0.1". COAST (please at least give the correct URL!) is indeed exciting, but interferometry is very technically challenging and increases somewhat exponentially with the number of apertures (at least in the optical). We're a long way off VLA-type large-scale optical interferometers. Please don't portray this as being in common usage - it's not (yet).
- MCAO and AO (adaptive optics) systems also do not compensate - again, resolution is great, but the field of view just sucks arse:-( Technical challenges are also significant for adaptive optics.
- On the infrared band, yes ground-based observations are blocked. But then, so are the UV observations that can be made from HST. Much science is left to be done here - some species only have observable transitions in the optical/UV, meaning we can only measure their metallicities in this wavelength band.
JWST and hubble are complimentary, not competing. Some of the science might overlap, but certainly not the greater part. Hubble is old, yes (and now dead, thanks to o'keefe and his political agenda), but it isn't obsolete at all.
Now if you want to argue about where to best put the resources....that's a whole new kettle of fish.
it's not all about pixels. The HST CCDs are "scientific" grade chips, which means they are MUCH lower in noise, bad pixels etc than your average digital camera. These requirements also make the chips hellishly expensive, since the yield of good chips is very low.
For a normal camera, there are gobs and gobs of photons all over the place. the HST CCDs routinely deal with only a handful of photons (1000s), so the requirements of low-noise are much more stringent.
yes, but there are also problems with laser guide stars. Like they're hard to get working. Also, when you start adding multiple guide stars, as in Multi-conjugate AO (MCAO), you decrease the field-of-view (which on a large telescope is already pretty small) with every laser guide you use. There are trade-offs. You get excellent images, but only over a tiny area.
Laser systems are also extremely complex (and hence expensive). You'd need to make a pretty good science case for why they're necessary, especially given that the *median* seeing in the antarctic (dome C) is already as low as 0.27" (and less than 0.15" for 25% of the time). Compare this to mauna kea (the current best site in the world) which gets to 0.4-0.5" on a good night.
Also, I think you might have missed the point with CCDs. without closing the shutter, you can't just discard photons from a ccd one they're detected. since there's no time-tagging (as in, say, the FUSE UV detectors) you can't exclude photons after the fact - "discarding those timeslots" is a bit harder than it sounds.
errr...NO! this has been said elsewhere but...the southern hemisphere is the most interesting astronomically, because it has a larger portion of interesting objects to look at. Things such as the bulge of the milky way; the majority of the plane of the galaxy; the south galactic pole; the large and small magellanic clouds; the sagittarius dwarf galaxy which is currently being destroyed as it falls into the MW.
The southern/northern hemisphere thing has been way over-emphasized here. Every earth-based telescope has declination limits - you work around them.
And finally, many of the largest telescopes in the world are "down there". Just because you live in america, doesn't mean you're not allowed to look at the sky in the south. A lot of research is being done in the US and europe, and a lot of it concentrates on object and facillities accessible only in the southern hemisphere.
The same way other AO systems work - by using nearby bright stars. This is a 16m dish - there are quite a few things considered "bright" by those standards
Slashdot Mirror
ZOMG! that's awesome! I've been using emacs for years, and never quite figured this out! SO helpful. no more saving things to a file called "foo" and dropping to a terminal to run wc, etc. :)
also...rectangles are the shit! every time I show friend some emacs, this is one of the first things i give 'em. they never cease to be amazed :)
For the interested, here is an online video of a presentation given by ken sembach, the HST project scientist, at a symposium earlier this year. In it, he describe the servicing mission (SM4) in detail, with a particular emphasis on the new instruments being installed (WFC3, COS) and those being repaired (STIS, ACS).
There some cool shots of the astronauts in the massive water tank that simulates zero-g, practicing removing all those screws with the specially designed screw-plate.
http://www.stsci.edu/ts/webcasting/ram/HubbleFellows2008/KenSembach031108Hi.ram
Runtime is 38:51
The inital "spark" is provided by gravity. When a star forms, it is from a slowly collapsing gas cloud. As the gas contracts under gravity, it heats up, eventually reaching a temperature for fusion to begin. The temperature is important, because the hydrogen nuclei (i.e. protons) normally repel each other...thus they need a fair bit of energy to overcome the repulsion, and get close enough to "stick"...i.e. fusion.
This is very similar to the ideal gas law, PV = nkT....i.e. as the pressure goes up, there must be a corresponding increase in temperature. A cool experiment to illustate this is to put your finger over the pointy end of a fat syringe, and then press really hard. The increased pressure will heat the gas. This is also way scuba tanks need to be filled in a bath of water...compressing the gas to put into the tank creates a fair amount of heat.
There is a little confusion about how the elements are created, and where HE1523 got all it's metals from...so here is a quick primer on the way things work.
The big bang forms hydrogen, dueterium, some helium, and a tiny amount of lithium. In fact, the theory of what should be formed (called Big Bang Nucleosynthesis), and what is observed, agree incredibly well.
Most stars just burn hydrogen into helium, fusing the two hydrogen atoms. More massive stars burn hotter, and so they can ignite helium burning, forming carbon, nitrogen, oxygen etc. The hotter the star gets, the heavier things can be fused, all the way up to iron. All of these processes *release* energy, if you can get it hot enough to start the reaction.
After iron, to make heavier elements you have to *put in* energy, so the way elements are formed is different. Instead of fusing two things together, you now just add a single neutron to the nucleus. This is a very different process (called neutron capture)...and can happen veeeery slowly (in stars) or very rapidly (in supernova explosions).
So, uranium and thorium are both elements which are made in the rapid process (r-process) -- they are only made in supernova explosions...because in a supernova, the neutron density is very high, so catching one is more likely.
Anyway...the point of all this is that, by observing uranium, we KNOW there had to have been at least one dying star going supernova, which made the uranium. Then that gas collapsed again later, to make anna's star.
So far, no-one has yet managed to find a first-generation star, but it's a big area of research at the moment, and is one of the things anna is trying hard to find. By looking at these very old stars, we get a good picture of how a supernova works, because we see the product of ONLY ONE of them. With young stars, there might have been hundreds, all polluting the gas at different times...and disentangling that is really tough.
As for the age of the universe, WMAP told us that very precisely -- 13.7Gyr (with an error of only ~0.1Gyr). The age we derived from HE1523 is much less precise...but nucleocosmochronometry (stellar age dating), is an incredibly tough thing to do, but it does offer independant confirmationg of the WMAP result.
That's correct. The star is metal-poor -- it's has an iron abundance (the standard measure of how much metals a star has) of [Fe/H] = -2.95. This is a lograthmic scale, and means that, on a scale where the sun is 0.0, HE1523 has about 1/1000th the amount of iron. The bracket notation means [Fe/H] = log10{N(Fe}/N(H)} - log10{N(Fe)/N(H)}_sun...i.e. the logarithmic difference of the number of atoms of Fe, compared to hydrogen, normalised to the solar ratio.
But the kicker is that HE1523 is very heavily r-process enhanced too...which means that it has a lot r-process, neutron-capture elements (think Uranium and thorium), compared to how much iron it has. HE1523 has [r/Fe] = 1.8....which means it has a 100 times more r-process heavy metals compared to iron, than does the sun.
BOTH of these factors are very important for this measurement, because you need to have very few metals, very high signal-to-noise data, very high resolution, and very strong r-process abundance, in order to be able to observe the uranium line. Anna needed 7.5hrs of VLT time to get a signal-to-noise ratio of about 350 or so...much higher than the S/N ~ 50-75 that we got from Magellan.
You can get a pdf of the paper here. Check out Fig 2, which shows the relevant part of the spectrum, with the Uranium line. See how it's right next to the booming Fe line...that's why we need a low iron abundance to do this work.
Complete bullshit.
Your cost estimates are accurate, but the rest of your argument is total shit. Adaptive optics, WHEN it works (which is rarely, and with difficulty), can approach the angular resolution of HST in a VERY SMALL field of view. You cannot get 0.05 arcsec, diffraction limited images over a wide field of view, that is possible with HST.
"Designing a mirror to withstand a launch vehicle" is a problem that has been solved. And the only two current, viable telescope proposals for telescopes larger than 10m are the Thirty Metre Telescope (TMT) and the Giant Magellan Telescope (GMT). OWL is not a concept that is being taken very seriously...ESO certainly hasn't put its money where its mouth is.
Your final point, about not many lines in that part of the spectrum, belies a complete lack of understanding of what you are talking about. The UV (accessible with STIS, and the Cosmic Origins Spectrograph, which will fly on SM4 in late '08) are so full of lines that they overlap all over the place. See, for example, Morton (2003), ApJS, 149, 205, for a comprehensive list. At low redshift, lines of HI, OI, OVI, CIV, NV, CII, SiII, SII, FeII, NI...all are in the UV, in the STIS band. Furthermore, space is the ONLY place these wavelengths can be observed, because of the atmosphere is opaque to wavelengths shorter than about 3300 angstroms.
JWST will provide diffraction-limited images at 2 micron. It will have imaging and spectrographic capabilities in the near and mid-IR -- everything from 6000AA out to 27micron with the mid-IR imager and spectrograph (MIRI). StSci has a JWST primer online here (pdf link).
complete bullshit. This has nothing to do with dark matter, or the amount of visible mass. It merely tells you what percentage of the sun is comprised of oxygen. It is a very tiny amount. For every oxygen atom, there are about a 1000 hydrogen atoms and a hundred helium atoms.
The evidence for dark matter is based on other observations, like the way disk galaxy's rotate. In order to reproduce those observations, dark matter is required.
The estimate of the total amount of mass in various phases (e.g. stars, cold gas, hot gas, etc etc) in the universe has been done. You can read the paper here. Look at table 1. This is the contribution of all the different things to the total energy-density of the universe. What is amazing is the *tiny* fraction of the total energy-density that is made from baryons (visible, observable stuff). It's only about 4% or so. 23% is dark matter, and the rest is "dark energy".
This is not a new issue in astrophysics, and has been floating since 2004. There are two basic ways to measure the abundances. One is by looking at hte oscillations in the sun, and using those to probe the solar interior. This is called "helioseismology", since it is very similar to the way seismologists figure out the structure and composition of hte earth, by observing seismic waves.
The other way is to take a spectrum of the sun (which is really just the solar photosphere -- the outer layers, or "atmosphere"). To interpret the spectra, one needs a model, which is used to derive the abundance (how much oxygen there is).
Now...until recently the models used for deriving abundances were simple 1-dimensional models, which made some assumptions (such as "local thermodynamic equilibrium") and include some fudge factors to account for the fact that you're solving a 3-d problem in 1-d.
The oxygen problem arises when you use accurate, 3-D models, which don't make the LTE assumption mentioned above -- called non-local thermodynamic equilibrium (NLTE). When one compares the abundances from the 3d NLTE models with what is expected from the helioseismology predictions, the discrepancy arises.
Others have posted the link to the full journal article on the pre-print server (here). The introduction of this paper is a pretty good summary of the problem, albeit intended for a scientific audience.
Yup. There was a paper a few years back entitled "terascale sneakernet", by jim gray and a couple of guys at MSFT research division on this. You can find it in the arxiv.
This concept has also been applied to such things as the Sloan Digital Sky Survey. Astronomers do tend to generate a lot of data with large surveys such as this.
actually, there is a grant flow-on from hubble operations, in that observing time on HST can translate directly into money (e.g. HST General Observer grants).
Further, HST has made many observations that are simply not possible from the ground, even with 8m-class telescope and adaptive optics (which are notoriously difficult to get working). E.g. observing in the UV is simply not possible from ground-based telescopes.
sorry, wrong again. HI, as defined and used by every astronomer on the planet, it neutral hydrogen. That's a H with a roman I next to it. HII is ionised hydrogen (H+ to chemists). H_2 is molecular hydrogen.
1) You definitely wouldn't see single stars. We'd see only the integrated light from a whole population of stars.
2) The numbers are already done for us. From the paper: 'We conclude that there is no optical counterpart to VIRGOHI21 down to a B-band surface-brightness limit of 27.5 B mag/arcsec^2. This is less than 1 solar luminosity pc^-2, giving a maximum luminosity in stars of less than 10^8 solar luminosities if a diameter of 16 kpc is assumed.'
3) M31 isn't far away at all. In fact, its the closest large galaxy to the MW. HST can resolve individual stars there, allowing us to measure the brightnesses and construct helpful "colour-magnitude diagrams" for instance.
4) No. Read the paper. They argue that the low surface density of gas prevents fragmentation of hte gas, and hence stars not forming.
5) This is total crap.
The answer, as with most things is, it depends. As I said, the MW halo is a good example. the stars in the halo are almost purely pressure supported with almost zero net rotation (most measures are 10km/s). But this is decoupled from the rest of the galaxy. Clearly the more things collapse, the faster they will spin...hence the rotation of the disk (circular speed of about 220km/s at the solar radius). The MW bulge I'm not so sure about (could look it up...).
In general there will always be some net angular momentum...whether it is significant is really dependant on the formation history of the object.
More detailed information can be found in the paper, which has been accepted for publication in a letter to the Astrophysical Journal.
Find it here.
errr...no, not really. Galaxies can also be "pressure supported"...in which case the stars have totally random motions. Each star has sufficient kinetic energy to prevent it falling in, but none of these velocities need be aligned. Thus all the stars are going in random directions, like atoms in a gas (hence the pressure reference).
In fact, stars in the halo of the milky way is pressure-supported with little or no bulk rotation. gas behaves quite differently, of course, since instabilities can form much more easily.
They observed the neutral hydrogen gas (HI), which emits radio waves at the well-known 21cm wavelength. This is not dark at all. From the rotation of the gas, we can work out, with a few assumptions, how big the gravitational potentional would be required in order that the gas is bound. This extra mass is assumed to be dark matter.
NOT TRUE!! The difference between the stars LSR and GSR velocities is similar from out point of view, because it's velocity vector points almost perpendicular to our motion around the galactic centre (see my other comment for more details about frames).
For observers 1/4 of the way around the galaxy in the disk at the same radius, it will be vastly different, since then the circular velocity aroudn the galactic centre is in the same dircection (roughly) as the star and so the difference between "LSR" and GSR frames will be significant.
All measurements of stellar velocities are not the stars true velocity - they are "radial" velocities. This is the geometric projection of the stars 3 dimensional velocity along the line of sight. Now, when you do this, everything is moving and many corrections must be made.
First, we correct for the rotation of the earth, and the movement of the earth around earth-moon barycentre. Then we correct for the earths movement around the sun. This is what is usually reported, and it is called the Heliocentric (i.e. centred on the sun) radial velocity.
Now the sun moves a little bit (about 20km/s) with respect to stars in our local neighbourhood, so we correct to this so-called "local standard of rest" or LSR.
From the LSR velocity, we can take out the movement of the sun aroudn the galactic centre, and put it in a galactocentric standard or rest, or GSR. This is done via the relation
v_GSR = v_LSR + 220*sin(l)*cos(b), since the sun has a circular velocity around the galactic centre of about 220km/s. l and b here are the galactic latitude and longitude, with (l=0,b=0) the galactic centre and l increasing counter-clockwise.
So, what we really care about, is the GSR velocity, from which we can tell whether the star is bound or not (i.e. is v_GSR > escape velocity).
There is a link to the actual article here (which has been submitted).
BUT! Don't forget, we measure only 1 component of the velocity (towards/away from us) - we really need all 3 (side-to-side as well) which are called "proper motions". So it could in fact, have higher a higher velocity.
HTH
Sorry, but many of the points you make are correct, but you basic premise - that HST is obsolete, is wrong. For these reasons:
:-( Technical challenges are also significant for adaptive optics.
- Not all of "us" are asking the same questions - HST is incredibly versatile. I was waiting for COS to go up in the next service mission, since it was going to have a huge impact in my field, studying the possible accretion of gas into the milky way.
- Interferometry is only applicable in specific circumstances and is not a general replacement for diffraction-limited imaging at resolutions of about 0.1". COAST (please at least give the correct URL!) is indeed exciting, but interferometry is very technically challenging and increases somewhat exponentially with the number of apertures (at least in the optical). We're a long way off VLA-type large-scale optical interferometers. Please don't portray this as being in common usage - it's not (yet).
- MCAO and AO (adaptive optics) systems also do not compensate - again, resolution is great, but the field of view just sucks arse
- On the infrared band, yes ground-based observations are blocked. But then, so are the UV observations that can be made from HST. Much science is left to be done here - some species only have observable transitions in the optical/UV, meaning we can only measure their metallicities in this wavelength band.
JWST and hubble are complimentary, not competing. Some of the science might overlap, but certainly not the greater part. Hubble is old, yes (and now dead, thanks to o'keefe and his political agenda), but it isn't obsolete at all.
Now if you want to argue about where to best put the resources....that's a whole new kettle of fish.
it's not all about pixels. The HST CCDs are "scientific" grade chips, which means they are MUCH lower in noise, bad pixels etc than your average digital camera. These requirements also make the chips hellishly expensive, since the yield of good chips is very low.
For a normal camera, there are gobs and gobs of photons all over the place. the HST CCDs routinely deal with only a handful of photons (1000s), so the requirements of low-noise are much more stringent.
cow milk is 87% water. Top link on google for "milk composition".
I thought even the trolls were able to use google these days...
yes, but there are also problems with laser guide stars. Like they're hard to get working. Also, when you start adding multiple guide stars, as in Multi-conjugate AO (MCAO), you decrease the field-of-view (which on a large telescope is already pretty small) with every laser guide you use. There are trade-offs. You get excellent images, but only over a tiny area.
Laser systems are also extremely complex (and hence expensive). You'd need to make a pretty good science case for why they're necessary, especially given that the *median* seeing in the antarctic (dome C) is already as low as 0.27" (and less than 0.15" for 25% of the time). Compare this to mauna kea (the current best site in the world) which gets to 0.4-0.5" on a good night.
Also, I think you might have missed the point with CCDs. without closing the shutter, you can't just discard photons from a ccd one they're detected. since there's no time-tagging (as in, say, the FUSE UV detectors) you can't exclude photons after the fact - "discarding those timeslots" is a bit harder than it sounds.
errr...NO! this has been said elsewhere but...the southern hemisphere is the most interesting astronomically, because it has a larger portion of interesting objects to look at. Things such as the bulge of the milky way; the majority of the plane of the galaxy; the south galactic pole; the large and small magellanic clouds; the sagittarius dwarf galaxy which is currently being destroyed as it falls into the MW.
The southern/northern hemisphere thing has been way over-emphasized here. Every earth-based telescope has declination limits - you work around them.
And finally, many of the largest telescopes in the world are "down there". Just because you live in america, doesn't mean you're not allowed to look at the sky in the south. A lot of research is being done in the US and europe, and a lot of it concentrates on object and facillities accessible only in the southern hemisphere.
The same way other AO systems work - by using nearby bright stars. This is a 16m dish - there are quite a few things considered "bright" by those standards