Light Echoes Solve Mystery of Tycho's Supernova

Ponca City, We love you writes "Powerful telescopes in Hawaii and Spain are using 'light echoes' from the original supernova explosion that have bounced off dust in the surrounding interstellar clouds to identify the precise type of supernova that Tycho Brahe saw 436 years ago. Although the echoed light from Tycho's supernova is around 20 billion times fainter than the original light observed in 1572, the team took identical images of the sky a few months apart and then digitally subtracted one from the other to find evidence for several sets of light echoes rippling across patches of dust in the northern Milky Way. 'Using light echoes in supernova remnants is time-travelling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event. We got to relive a significant historical moment and see it as the famed astronomer Tycho Brahe did hundreds of years ago,' said Tomonori Usuda, of the Subaru Telescope in Hawaii. Tycho's original observations were particularly important as he immediately concluded that the new star, visible even by day, could not be closer than the Moon challenging the Aristotelian view of the cosmos, widely accepted since ancient times, which held that the sky beyond the Moon never changed."

Comments

I want to play.

[suso]

That is really cool. Like some kind of galactic diff.

Wikipedia links just for the sake of completeness

[Anonymous Coward]

Here's a link to the supernova in question. Also, here's Brahe himself. Remember that all his observations were naked-eye - pre telescopes.

Re:Light echoes?

[Vellmont]

Is there something wrong with the word "reflections" now?

In human experience, a reflection is instantaneous, where an echo appears after a period of time. Thus echo is more descriptive to layman (remember them?). You know and I know that a reflection isn't instantaneous, it's just not generally perceptible to our eyes like an echo is perceptible to our ears.

A galactic yardstick?

[mangu]

This could be used to determine distances very precisely. If we know when that light was emitted and we know the speed of light, then we can calculate with great precision the distance from the star to the reflecting dust cloud.

Re:A galactic yardstick?

[Timothy Brownawell]

This could be used to determine distances very precisely. If we know when that light was emitted and we know the speed of light, then we can calculate with great precision the distance from the star to the reflecting dust cloud.

You also have to account for any differences between the earth-star distance and the earth-cloud distance, but it's still the largest ever radar system.

Re:A galactic yardstick?

[mangu]

You also have to account for any differences between the earth-star distance and the earth-cloud distance

One could start by assuming that the points which are being illuminated now and have the biggest angular separation from the star are at the same distance from earth as the star. Those points form a circle with a 436 light-year radius. The size of that circle as seen from earth will give you the distance to the star.

I'm assuming that there is enough dust everywhere in space to return a detectable reflection, but even if this isn't true an imperfect circle would still give us usable data. At the distance that star is, about 7500 light-years, this would probably more accurate than other methods.

Supernova 1987A has had its distance measured by a similar method, look in this picture how the reflections appear.

Re:A galactic yardstick?

[halcyon1234]

Although... if we know when that light was emitted, then we know the distance to the supernova already. There's four pieces of information that could be known. If we know two of them, we can map out the Earth/Supernova/Cloud system:

1. The distance from Earth to the Supernova at the time of the supernova event (equal to the time it took for light from the supernova to reach Earth)
2. The distance from the supernova to the reflecting cloud (equal to the time it took for the light from the supernova to reach the cloud)
3. The distance from the reflecting cloud to Earth (equal to the time it took for the reflected light to travel from the cloud to Earth)
4. The angle between the cloud and the supernova, with earth at the origin (projected onto two dimensions of your choice, for convenience)

5. 1. We know 1 is about 7500 light years. I can only presume that the smart brains behind the telescopes can tell determine 3 from looking at the cloud. However, if they can already position the cloud, then you don't need to know 2, you can work it out even without the reflected light

      The hard part, I would assume, would be doing the math on not where everything is, but where everything was when the light hit, and adjusting accordingly.

Re:Light echoes?

[Sockatume]

"Echoes" evokes the idea that the light from the star first reaches us directly, then a delayed reflection of that light reaches us afterwards. "Reflections" are colloquially assumed to be instantaneous. I think it's a neat bit of semantics, really.

Reverse Ray Tracing

[Doc Ruby]

When can we point our telescopes at an object hundreds or thousands of lightyears distant, and pick up the light reflected back at us that previously traveled from Earth to that object, then reassemble it into images? Images of the Earth's past, twice as old as the lightyear distance of the object?

We could look at an object 1000 lightyears distant for reflections of Jesus being crucified. Search among objects 250-600 lightyears distant for reflections of people arriving in the "Americas" on ships before Columbus. 176ly distant objects could show us images of Newton getting hit by a falling apple.

Finally a use for the combined computing power of all Earth's computers.

Re:Reverse Ray Tracing

[glaswegian]

We would need monumentally large telescopes for this. The largest optical telescope on earth, the Keck, has a diameter of 10m. Using the Rayleigh Criterion, we can calculate the minimum resolvable detail at a given distance. For example, we can resolve details on the surface of the moon (in the visible) that are around ~20 meters across. If you want to resolve an apple falling on somebodies head you need ~10 cm resolution. So to see this happening on the moon we would need a telescope with a primary mirror ~ 2.6 km across. To see the same thing echoed back from a dust cloud near the closest star to our sun (4 ly * 2), you would need a telescope with a primary ~ 7e+10 m across or around half the distance between the earth and the sun. Not to mention that the signal would be very weak and completely lost in noise.

Re:Reverse Ray Tracing

[Doc Ruby]

A telescope array acting as an interferometer doesn't need to be a single large sensor like that. We can orbit the array with separations of 1E13, just beyond Neptune. That would give us resolution of something like 5.8E-20 arcseconds. The radius of that regular polygon with 10cm sides is about 7E21m, or about 740,000 light years. Which would show light that left Earth about 1.48 million years ago. Orbital arrays much closer to Earth are sufficient for looking for apples only 175ly away.

The signal to noise is of course extremely high ("astronomical"). That's why I mentioned the combined computing power of all the world's computers. We're gonna need a bigger boat, but that's a good sea to sail her on, to catch this shark :).

Re:Reverse Ray Tracing

[Urkki]

It's not just computing power. We'd actually need to catch enough reflected photons that originated from the Earth, so we'd have anything at all to process.

I think it would be a problem even if there was no other light except what we want to observe, ie. there would be no external noise.

Also, the "mirror" isn't a flat plane, so we'd get a bunch of "Earth photons" that originatead at different times, reflected at different times, and then arrive at our telescope at the same time.

To get anything useful, we'd need to have a very sharp focus, and then move the focus at the speed of light, so we'd collect photons that originated at the same time, but reflected at different times.

So sharpness of focus would determine temporal resolution. If we'd want to catch an apple falling, we'd need temporal resolution of maybe 1/10th of a second (and that would still be quite a blurr), so focus would have to be 1/10th of a light second... I don't know that much about optics, but achieving that at a distance of hundreds of light years would be quite a feat too... Will a telescope array help with achieving a sharp focus at a great distance?

Re:Reverse Ray Tracing

[AlejoHausner]

This would definitely not work. There is no imaging (no lens, or pinhole) at work. What you are proposing is analogous to sitting in a dark room with white walls with the television on: you will see areas of color on the wall, but you will be unable to deduce from these reflections what the picture on the TV looks like (beyond getting the average color of the TV picture). Of course, if the wall were a mirror, you could do it, but walls are diffuse reflectors, which means that, at each point on the wall, any light arriving is scattered into a 180-degree hemisphere of directions. The light reaching your eye from any point on the wall comes from the whole TV screen. You can't easily undo what is, effectively, a massive blurring operation.

To make matters worse, room walls are white, and reflect light back and forth to each other, so a large fraction of the light you see on the wall has already bounced off one or more other walls. Thus it's undergone several blurring passes.

This latter inter-reflection problem will, of course, happen in space too. There's lots of nebula to scatter light.

And, what's worse, we don't know exactly where those nebular are, at least not precisely enough to get the data needed to undo the blurring, assuming that were at all feasible.

Alejo

Tycho Brahe - Amazing

[Aladrin]

I'm amazed that he was able to observe that and figure out that the common concept of the sky was wrong at the time. I can't imagine how much thought must have gone into something like that.

Re:Tycho Brahe - Amazing

[Aliks]

Well he had already done a lot of work on parallax measurements for astronomical objects. So when the supernova appeared, and showed no parallax against the moon . . . he was on pretty firm ground stating that the moon was closer to earth than the supernova.

More details in Wikipedia.

Re:Tycho Brahe - Amazing

[lawpoop]

It is laudable, but I wonder how much doubt there was going on at the time. For instance, the Greeks knew that the Earth was round, but there was common conception that the Earth was flat -- I'm not talking about what educated people thought, but what the commoners thought. European scholars studied classical literature ( back then it was just "literature" I guess ), and they were exposed to ideas from the Greeks and the Muslims, who helped transmit the Greek texts. So they got exposed to a lot of different ideas.

The official line of the church was that the heavens never changed and were perfect, and if you wanted to be in the good graces of the church, which was a good idea to a lot of people, they towed the line. Of course, to the common person, the sky was a round dome, because that's what it looks like, and that's what the priest tells him. But I think that the educated class might have been more open-minded.

Anywho I have a friend who claims that our perception of the universe is wrong -- we're basically looking at a big optical illusion that also affects gravity somehow. He doesn't claim to know what it should really look like, but he says what we are seeing is an optical illusion. I can kinda see his point -- all the instruments we have are earth based, and if there's some uniform membrane or something around the solar system, how could we tell? I don't know enough to prove him wrong, so whatever.

Doubting Thomas

[cidhawk]

Tycho: I saw a light in the sky that looks like an exploding star.
Scientific community: We don't believe you until we can see it ourselves. Neh!

Re:Doubting Thomas

[mrsquid0]

Actually, the scientific community of the time (such that it was) was mostly convinced by Tycho's observations.

Re:Light echoes?

[Yvan256]

HA-ha! You like semantics! /Nelson Muntz

Re:Light echoes?

[nschubach]

That's alright, I'm still trying to figure out which way is 'North' in space... Does North always point to the magnetic pole of Earth even on Mars? Has someone studied the Milky Way and determined that there's a magnetic ring perpendicular to the dish?

Humankinds' endless fascination with the sky...

[Tastecicles]

...continues to bring surprises like this. I'm just wondering if this is the same method we astronomers use to detect local masses such as transneptunian planets (or "Plutoids", if you will) or asteroids or -gulp!- Near Earth Objects such as the Saturn V Stage discovered and misidentified as a natural coorbiting body a couple years ago? Could light ripples be detected and identified on a pair of plates of the same patch of sky taken a year apart?

Re:Light echoes?

[mrsquid0]

North in the sky is defined to be the point directly above the Earth North Pole of rotation. The northern half of the sky is the part of the sky between the celestial equator and the north celestial pole. For a planet north is defined using the right hand rule of rotation. Curl the fingers of your right hand. That is the direction of the planet's rotation. Stick out your right thumb. That is the direction of the planet's north pole. The same rule applies to Galactic north. Just apply the rotation rule to the Galaxy. Once you get outside the Galaxy supergalactic coordinates are used, which are defined here: http://en.wikipedia.org/wiki/Supergalactic_plane.

Re:20 billion times fainter?

[TheThiefMaster]

xxx times less than yyy == yyy/xxx.
It's common language these days, learn it!

Re:Light echoes?

[mrsquid0]

Hmm... I made a mistake. I should have said that supergalactic coordinates are described here: http://en.wikipedia.org/wiki/Supergalactic_plane

Re:20 billion times fainter?

[CrimsonAvenger]

But seriously, if something is 20 billion times fainter it's going to be barely visible, regardless of how bright the original is.

Our sun is ~20 billion times fainter than it will be when it supernovas. And seems to be bright enough to light up the world nicely. OP is right, it would be nice to know how bright the original was.

Re:Light echoes?

[osu-neko]

True. Actually, the real difference between the meaning of the word "reflection" and "echo" lies on in the delay but in the repeat. An echo is a reflection that is perceiving after one has already perceived the same thing the first time. So, although you see the gun fire and a second later here the report, that first hearing of the report is not an echo. But when you then hear the same report reflected off of the wall behind the guy firing the starting pistol, that is an echo. Since we saw the original supernova, then saw the reflection after, the second perception is of an echo. The delay is less important to the distinction than the fact that it's a repeat.

More substantial link

[AlejoHausner]

Here is a link to an article by one of the researchers involved in this work

http://www.naoj.org/Pressrelease/2008/12/03/index.html

As the article suggests, the biggest benefit of using light echoes is that the SPECTRUM of the original supernova can be obtained. In other words, while today we mostly see the direct-path light emitted by the supernova's gas remnant, light echoes let us see all the wavelengths of the light emitted at the time of the explosion.

Alejo

Re:20 billion times fainter?

[ScentCone]

Another few million years and Sol will just be a lump of cold nuclear waste.

Hey, don't sugar-coat it, OK?