Seeing stars in other galaxies

[bendiddley]

Something I have wondered recently is whether it is possible to see individual stars in other galaxies, not for us amateurs obviously because our scopes are two small but for the hubble maybe? I've tried googling this and some people say hubble can and some say it can't. What do you guys think?

[Knight of Clear Skies]

Yes, it's possible. Edwin Hubble identified a Cepheid variable star in Andromeda in 1923, making it possible to estimate its distance and settling the question of whether it was part of the Milky Way or a separate galaxy. That one measurement vastly increased the size of the known universe. Hubble used the 100 inch Hooker Telescope, which has a mirror about the same size as Hubble's.

[Laurie61]

I had not really thought about this, I would think the Hubble would not have enough resolution? We can of course see supernova in other galaxy's although not exactly the same thing       

[Knight of Clear Skies]

Here's one of the discovery plates of that famous Cepheid:

It's the one near the top of the plate, Hubble has crossed out N (for nova) and written VAR! (for variable) in its place. Image taken from the Carnegie Institute site.

[IanL]

Try this link for some examples of Hubble resolving (many) rather ordinary individual stars in Andromeda:

http://www.spacetelescope.org/news/heic1112/

It is also possible to image blue supergiants stars using rather more modest equipment .  Here's one I prepared earlier:

Look top left and you will see a blue patch with lots of indivdual bright stars which is NGC 206, but there are plenty of others on the outer arms elsewhere particularly to the lower right.

I've also read some posts on Cloudy Nights with people reporting to have seen some of the supergiants visually with 16 and 18 inch scopes.

[bendiddley]

Thanks guys, some interesting responses and some amazing imagery too which is pretty cool. I still can't understand though how something so small could stand out from the galaxy. I mean andromeda is huge and these stars are tiny.

[Knight of Clear Skies]

I think it's because these stars are very bright (Cepheid variables are up to 100,000 times as luminous as our Sun), and the average distances between stars in the spiral arms is very large. If you look at the full resolution hubble image of M81, in some places it's possible to look through the sparsely populated arms and see more distant galaxies behind them. I imagine picking out individual stars in the core would be much more challenging.

[IanL]

Thanks guys, some interesting responses and some amazing imagery too which is pretty cool. I still can't understand though how something so small could stand out from the galaxy. I mean andromeda is huge and these stars are tiny.

It is down to luminosity. Type OB supergiants are 10,000 to a million times more luminous than the Sun, so they can be picked out as individual objects even over a distances of a couple of million light years.

The Hubble telescope images I linked above are from the outer arms, where the stars are more sparse enabling it to resolve much less luminous stars.  You couldn't take an image near the core and resolve individual stars as they are much more tightly packed and appear to merge in to a single glowing source.

The breakthrough was that Edwin Hubble managed to identify an individual Cepheid variable, which has a well understood relationship between its absolute magnitude (true brightness) and the period over which the the magnitude varies.  The relationship was discovered by Henrietta Swan Leavitt in 1908 by investigating a large number of variables in the LMC, (with follow up by others) so that by 1924 Hubble had a solid means to determine the distance to the one he found in Andromeda.  (Basically measure the period of the variable, which tells you how bright it should appear, compare that to the measured brightness and you can calculate the distance from the difference between true and apparent brightness).  The physics causing the variability were discovered later, but basically the Cepheids change size in regular pulsations (by a very large amount in some cases) and the period is directly related to their size and temperature (thus brightness).

[ronin]

Don't think that individual stars can be resolved, the seperation is too small and if not on the edge the whole lot would basically merger into a blob in the scope, looking at the milky way demonstrates this. If we could pick out them individually then I can see no reason for Gaia that was recently launched - the stars in the milky way are a lot closer then the ones in M31.

Seeing something brighten like a cephid variable does not mean isolating that specific star, the brightness of a small patch will alter with the cephid.

Other option is to sork out a "typical" seperation, say 5 light years and at 2.5 million light years away determine the angle and then compare to the resolution of something like Keck.

[Moonshane]

they are certainly visible as supernovae

[swamp thing]

Remember reading an account of some amateurs getting to look through the 200" on mount palomar in astronomy magazine, many years ago. The report spoke of resolving individual stars within the andromeda Galaxies core. They said the core appeared like an enormous globular cluster.

Whether this was just excited imaginations I've no idea. Seeing as andromedas own globs appear non stellar in even amateur kit I would think with a 200" scope it could be possible to resolve some stars.

[bendiddley]

they are certainly visible as supernovae

You're not wrong! It was amazing seeing the supernova in m81 earlier this year, and funny enough that is what got me thinking about this in the first place. Thanks everyone for your responses by the way, that's amazing to know that it is possible. Ok but one more question, how far before it is impossible. Andromeda is 2 million light years, so how much further than that can our technology resolve stars?

[dph1nm]

It is trivial to resolve the outer parts of M31 with HST - indeed individual globular clusters in M31 can be resolved. There are some nice colour-magnitude diagrams out there.

NigelM

[crashtestdummy]

Some of clusters in m31 are detectable with amateur,albeit large amateur telescopes but actually resolving individual stars isn't something you will be doing from your back garden.

[bendiddley]

Some of clusters in m31 are detectable with amateur,albeit large amateur telescopes but actually resolving individual stars isn't something you will be doing from your back garden.

Yes I know that but wouldn't it be great if we could.

[IanL]

Sorry but I don't understand all the assertions that you can't resolve individual stars in Andromeda when:

- I have posted an image above that clearly shows a whole bunch of OB supergiants that have been resolved in NGC 206 and elsewhere in the outer arms.  For sure the blue glow in those regions is due to lots of other less bright supergiants that haven't been resolved, and with a bigger scope and pixel scale you can resolve more and more of them, but the brightest objects are seen as individual stars.

- You have all seen pictures of recent supernovae in galaxies that are much further away.  If the star wasn't resolved then what are you seeing?

The basic point is that it is a matter of relative luminosity - a bright enough star amongst a large number of dimmer ones will be resolved as an individual object.  The Milky Way illustrates this perfectly.  Most of the stars that you can see in the sky belong to our galaxy.  If you image a portion of the MW you will see a bright background glow of unresolved stars, but on top of that you will see plenty of individual stars that have been resolved due to their greater brightness (either due to being closer and having a greater apparent magnitude, or by having a greater absolute magnitude or typically both).

The point about extra-galactic supernovae further illustrates it, since you can clearly resolve a supernova with a small scope that is tens of millions of light years away against a galaxy background of hundreds of millions of other stars.

There may be a small semantic point here that the individual objects also contain light from many, many unresolved stars in front of and behind it, but then the same can be said of pretty much any star in the sky (whether in the plane of the MW or not).  There is no dispute about the difficulty of resolving much fainter stars, for that you need the HST and sparser outer regions of the galaxy.

[crashtestdummy]

I was under the impression that the op was talking about seeing them visually which with most amateur scopes the clusters in m31 won't show you individual stars.a supernova is a different kettle of fish as you can't really class it as a star anymore

[IanL]

I was under the impression that the op was talking about seeing them visually which with most amateur scopes the clusters in m31 won't show you individual stars.a supernova is a different kettle of fish as you can't really class it as a star anymore

See here for assertions to the contrary:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6117601/page/0/view/collapsed/sb/5/o/all

[swamp thing]

See here for assertions to the contrary:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6117601/page/0/view/collapsed/sb/5/o/all

Very interesting Ian. Thanks for the link.

[IanL]

The posters in the thread above claim that they have resolved some of the super giants visually with equipment as small as 12" aperture (questionable perhaps), but certainly the 16" and 18" claims seem credible.

Following up on this, one of the links in the CN thread is to a photometric survey of NGC 206 here:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?letter=.&classic=YES&bibcode=1987AJ.....93..310O&page=&type=SCREEN_VIEW&data_type=PDF_HIGH&send=GET&filetype=.pdf

Table III (page 312-313) catalogues bright stars in NGC 206 and there is a chart marking out some of the brighter stars which I have reproduced below.  I have also marked the corresponding stars in my image from above.  As you can see there are plenty of individual stars that are luminous enough to be resolved (marked in RED) with an 80mm refractor (f 6.4, unmodified Canon 500D, 3 hours of exposures).  There are also a bunch  that cannot be resolved (marked in GREEN), not because they are too faint but simply because the aperture and pixel scale is insufficient so they merge with other nearby bright stars.  Now the author's criteria for membership of NGC 206 is a B-V of less than 0.5, and on that basis star number 6 is certainly not a foreground star (B-V 0.47).  22 is B-V -0.35 and is almost resolved, as is 60 (B-V 0.13),

Some more members might have been resolved with a triplet and a larger number of shorter exposures (e.g 5 and 12), since the 80ED and 1800 second subs creates a fair amount of star bloat. Assuming I can sort out my guiding, my 8" SCT would have no problem resolving them given sufficient exposure time.