Mysterious Stars Surround Andromeda's Black Hole

UltimaGuy writes to tell us that Yahoo is running a story about a recent discovery that shows the source of strange blue light coming from the center of the Andromeda galaxy. The light is actually a cluster of stars circling the galaxy's central black hole with immense orbital velocity. From the article: "Such frenetic activity was thought to prevent star formation. Stars form when a knot of gas and dust collapses under its own gravity."

Comments

Elements past iron

[benhocking]

Elements past iron can only be created in a supernova explosion. Google on "supernova elements" for more information. Of course, the element synthesis during a supernova explosion is due to fusion, but I'm not sure one could call it "star fusion".

Re:Elements past iron

[MaskedSlacker]

Actually its neutron accretion that produces elements heavier than iron, not fusion. Iron disintegrates at temperatures lower than what it will fuse at.

Re:Elements past iron

[MaskedSlacker]

No, because hydrogen is a single proton, not a single neutron. I forget the exact mechanics of neutron accretion, as its not my field of direct study, but it occurs in two forms, the r-process (rapid) and the s-process(slow) (we physicists are not known for being creative with names). The r-process occurs in supernovae when heavy nuclei are bombarded by many neutrons, ad rather than splitting the target nuclei, the neutrons stick (at the same time the nuclei are radiating particles away, but not as fast as they are gaining them). Once the process stops, the new, super neutron rich nuclei give off beta radiation (changing neturons into protons) until they reach a stable configuration. The s-process occurs in large, but otherwise stable stars. This process however only produces elements as heavy as lead. Anything heavier is produced by the r-process.

An escaping star?? HA!

[lightyear4]

Wouldnt matter too much...our Milky Way and Andromeda are on a slow collision course anyway - by the time an ejected star got here, the rest of the galaxy would be right behind it. But no need to go hide in a cave just yet, we've still got about 3 billion years.

Nasa site had this as well

[qray]

An artist's rendition on their picture of the day:

http://www.nasa.gov/multimedia/imagegallery/image_ feature_411.html
--
fu

Re:Well...

[swelke]

That's quite possible. If they formed in a wider orbit around the black hole, for example, they could well have been caught by tidal drag and slowly moved into lower orbits.

The real question is how they can exist at all in such a low orbit (or, more accurately, how they can exist in such a strong gravity gradient). What happens is that if they tidal difference between the two sides of the star (the difference between the black hole's gravity at the closest edge of the star and that on the furthest edge) exceeds the star's escape velocity, matter will be able to leave the star and it just falls apart. The implication (which the Yahoo! article was too low-tech to get right) was that the stars must be very dense. A dense star will have both (a) less distance between that nearest and furthest edge and (b) a steeper gravity well for material to get out of in the first place.

The other interesting bit is the rather close estimate of the black hole's mass. Most of the other estimates of galactic center black hole masses I've seen are based on things orbiting them far more distantly, such as 10-100 light years.

Re:Heavy elements

[cswiger2005]

The initial gas mix of stars is approximately 95% Hydrogen (H) and about 5% Helium (He), with a very tiny fraction of Lithium and heavier elements.  Anything heavier than Helium is considered a "metal" to an astronomer, BTW.

Stars produce the most energy by fusing H into He, and they can also gain some energy by fusing heavier elements, but the amount of energy declines until you reach Iron (Fe), after which fusion no longer results in an energy gain.  Once a star starts having a lot of Fe in its core, the fusion synthesis process stops producing the energy needed to keep the star going, so it collapses, and the rebound shock causes a nova, also producing elements heavier than Iron as a result.

The approximate distribution of elements in Sol, our sun, today is:

{'H':.785, 'He':.197, 'O':.0097, 'C':.004, 'N':.001, 'Si':.001, 'Mg':.00076, 'Ne':.00058, 'Fe':.00014, 'S':.0004}

Planets like Earth have a much higher distribution of heavier elements than stars do, for a very simple reason: they aren't big enough to have enough gravity to keep things like H and He from escaping over time, unless the planet is above a critical mass, in which case it forms a gas giant like Jupiter, Saturn, etc, which are big enough to retain such very light gasses.

Here's a table of the planets in our solar system, with mass measured in 10**24 kg, density relative to water, then the heaviest molecular weight of a gas the planet will retain, and the lightest common atmospheric gas which is kept:

Mercury  mass:    0.33 density: 5.43 moleculelim:  43.5 gas: CarbonDioxide
Venus    mass:    4.87 density: 5.24 moleculelim:   7.3 gas: Methane
Earth    mass:    5.97 density: 5.51 moleculelim:   6.3 gas: Methane
Mars     mass:    0.64 density: 3.93 moleculelim:  31.1 gas: OxygenGas
Jupiter  mass: 1899.00 density: 1.33 moleculelim:   0.2 gas: HydrogenGas
Saturn   mass:  568.00 density: 0.69 moleculelim:   0.6 gas: HydrogenGas
Uranus   mass:   86.80 density: 1.27 moleculelim:   1.7 gas: HydrogenGas
Neptune  mass:  102.00 density: 1.64 moleculelim:   1.4 gas: HydrogenGas
Pluto    mass:    0.01 density: 1.75 moleculelim: 578.5 gas: None

In particular, oxygen gas, O2, has a molecular weight of 32, and N2 is 28.  If Mars were just a little bit heavier, it would have a much more substantial atmosphere which would be much more similar to that here on Earth.

-Chuck

PS: Why yes, that's a Python dictionary above, you didn't think I'd write the table above by hand or post using "Code" frivolously, do you...?   :-)