Starship Asterisk*

Is it true, there are no stars in this picture ( APOD Oct. 17, 2007? )
Are all the points of light in the galaxies presented star clusters? And are all the other smudges and points of light outside of the galaxies other galaxies?

To me it looks like a normal galaxy formation which has been sliced or cut in two by another galaxy

http://antwrp.gsfc.nasa.gov/apod/ap071017.html

The smudges are thought to be galaxies. Individual stars would show up as very round dots without gradient outwards. Although globular clusters might appear as a point source at that distance?

One star or a group of stars? (segments taken from the image)

Some of the objects are stars in the Milky Way, and some are background galaxies or star clusters. But some are actually individual (or chance allignments of two) stars in the target galaxy. The HST is just about capable of resolving individual stars at this distance, but only if they are bright enough, the old stars they have found are so called Red Giant Branch stars which are very bright despite being quite low mass, because they have evolved into the Red Giant phase.

I have a hard time fathoming the sheer size of that molecular cloud. It spans such a huge distance, encompassing quite a few other galaxies. I don't see how this could be an illusion either as a lot of the redish galaxies are infront of the cloud.

I wonder if they're certain the older stars aren't there as a result of a merger with a much smaller, older galaxy or globular cluster. The younger stars definitely seem predominant.

astro_uk wrote:The HST is just about capable of resolving individual stars at this distance...

More precisely, it is capable of detecting individual stars at that distance if they are bright enough, and of splitting them if they are sufficiently separated. It is quite incapable of resolving any stars in other galaxies.

It's amazing that we've never actually seen a star other than our own Sun.

geckzilla wrote:It's amazing that we've never actually seen a star other than our own Sun.

That depends what you mean by "see". We have learned vast amounts about other stars by studying their light. However, I assume you mean "resolve"? In fact, a few stars have been resolved telescopically, allowing features such as hot and cool spots and limb darkening to be imaged. The first such star, and the classic example, is Betelgeuse.

Heh, yeah, I meant beyond a point of light to seeing the individual characteristics of the surface itself. I know a lot can be learned about a star by studying the light it emits but it still seems very abstract. The fuzzy image of Betelgeuse comes close but is still something a vast majority of people have a hard time appreciating.

I believe that Betelgeuse is appriximately the same diameter as Jupiter's orbit. If we were capable of imaging its surface with any clarity, we should also then be able to resolve Jupiter sized planets in jupiter sized orbits around closer "sun sized" stars.

Maybe, but planets don't emit light, so it would probably be much harder to spot them.

BMAONE23 wrote:I believe that Betelgeuse is appriximately the same diameter as Jupiter's orbit. If we were capable of imaging its surface with any clarity, we should also then be able to resolve Jupiter sized planets in jupiter sized orbits around closer "sun sized" stars.

It's still very difficult. 2M1207 and its companion planet have been directly imaged, with a separation of 0.8 arcsec (Betelgeuse has an angular size of about 0.1 arcsec). The planet is about 10 times farther from its star than Jupiter is from the Sun, and could be imaged because the star is cool and the planet is hot.

Trying to image a very dim planet which is close to a very bright star is a challenging task, but instrumental advances will probably make this possible any time. And again, we won't be resolving any planets, only splitting them from their companion stars.

Chris Peterson wrote:More precisely, it is capable of detecting individual stars at that distance if they are bright enough, and of splitting them if they are sufficiently separated. It is quite incapable of resolving any stars in other galaxies.

Not even the LMC or SMC?

bystander wrote:Not even the LMC or SMC?

Not even close. The Hubble is capable of a resolution of a bit better than 0.1 arcsec; larger ground-based telescopes using adaptive optics can do a little better than that. But that resolution, impressive as it is, only allows a few of the very closest, largest stars to be resolved, and just barely at that.

The Magellenic Clouds are 150-200 thousand light years away. Stars that we can hope to resolve are not more than a few hundred light years away.

As Chris has already pointed out, a lot hangs on what one means by 'see'.

If you require a single optical device to produce an image which is other than a point (which has its own meaning), then only Mira, Betelgeuse, etc (I'm not sure there is much of an 'etc') have been 'seen', as other than points.

However, the size - in (fractions of) arcseconds - of quite a few stars has been determined, as well as the way the intensity of light (as we see it) varies across the disc. There are several techniques for this, such as speckle interferometry, reconstructions from eclipses (by the Moon), the optical equivalent of long-baseline interferometry (Mt Palomar has a wonderful set-up, and the VLTs, in Chile, were designed with such interferometry in mind; there's also the descendents of the Narrabri intensity interferometer), and even 'starspot' reconstructions from the fine details of line profiles! Not to mention boring old eclipse binary reconstructions.

If anyone reading this is interested in more details of any of these, just ask, and I'll see what I can dig up.

Well, personally, I was only thinking about how impossible it is to get a good "image", like, say, the photos we have of close ups of the sun's sun spots, jupiter, saturn and saturn's moons, mars, etc... it's really superficial and not scientific at all. More of an insatiable interest than anything. If the variations within our very own solar system are so unique from planet to planet and moon to moon, one can only imagine what planets and stars really look like up close and how they are in a tactile sense.

geckzilla wrote:Well, personally, I was only thinking about how impossible it is to get a good "image", like, say, the photos we have of close ups of the sun's sun spots, jupiter, saturn and saturn's moons, mars, etc... it's really superficial and not scientific at all. More of an insatiable interest than anything. If the variations within our very own solar system are so unique from planet to planet and moon to moon, one can only imagine what planets and stars really look like up close and how they are in a tactile sense.

Then I think you'll find this 2005 living review very interesting: Starspots: A Key to the Stellar Dynamo!

You need to do some digging to find the 'pretty pictures', but there are some good links at the bottom of the page which is section 4.2 ('Doppler Imaging').

Among other things, you can see just how much is now known about the variation in appearance of stars (at least wrt starspots), for a significant subset of stars, even though there are direct, visual waveband, images of only a tiny number of stars ...

Nereid wrote:Among other things, you can see just how much is now known about the variation in appearance of stars (at least wrt starspots), for a significant subset of stars, even though there are direct, visual waveband, images of only a tiny number of stars ...

I've recorded starspots for dozens of fast rotating dwarfs, none of which are optically resolvable. As you note, we can learn all sorts of things about stars, including surface details, without the ability to actually image them as extended objects.