Supernova Factory in NGC 2770 (APOD 18 Jan 2008)

[hyperion]

I am somewhat new to this.......when the picture caption says an event "recently" occurred, does that mean we recently saw it or that it actually just occurred? If the galaxy is 90 million light-years away, does that mean that the recently viewed supernova occurred 90-million years ago?

[npsguy]

Using the word 'recent' really depends on the context in which it is being used. In the case of SN2008D in today's APOD of NGC 2770, "recent" means recently observed even though the event occured at somewhere around 90 million years ago.

When "Supernova 1987A" was first seen many articles in the press called it a "new" supernova even though it happened a 179,000 years ago because it is 179,000 light years away.

Just remember that if an event is "X" light years away we don't see the event until X years later. So even if someone says it is 'recent' they really mean 'recently observed'.

If an event is seen much closer, say a comet crashing into Jupiter or Comet Holmes flairs up. These 'local' events are normally 'recent' because they are observed minutes to hours after they really occur.

[hyperion]

Thanks...that cleared it up!

[Andy Wade]

http://antwrp.gsfc.nasa.gov/apod/ap080118.html

I think I've read, or heard at least that all resolvable stars that we can see are in our own Milky Way Galaxy, and that we cannot resolve a star in another galaxy as they're simply too far away.
Does this APOD mean that there are occasional exceptions to this rule?
If we can see the supernovae in NGC 2770 then are we actually seeing stars outside our own Milky Way galaxy?

[Case]

resolvable stars

In 1924, Edwin Hubble and Milton Humason resolved some individual stars in M31, using the 2.5 m telescope at Mt. Wilson. They were the first to use Cepheid variable stars to determine distance to spirals, thereby proving that these 'spiral nebula' were beyond our Milky Way.
For close galaxies some individual points of light can be identified as individual stars, if they are bright enough and stand out in some way.
Supernovae stand out by their huge brightness that wasn't there before and will fade in weeks or months.

[iamlucky13]

I just thought I'd add that the Hubble's sharpest camera has a spatial sampling (approximately the same as resolution) of 0.025 arcseconds per pixel, which works out to 0.3 light years at the distance to Andromeda.

Our sun, of course, is four light years from the nearest star, so in an environment like that, the Hubble could pick out individual stars, assuming it is sensitive enough to detect single stars, which I'm sure it probably is. Closer in towards the core, however, the density is much higher.

However, for almost any picture where you can recognize another galaxy as such, you can with very high confidence bet than any given point of light is a foreground star in the Milky Way or a cluster or supergiant star in the other galaxy.