Starship Asterisk*

As a non expert I wondered if anyone can help with a question of understanding this fantastic picture? Of the stars we can see are all of them part of the cluster, and if not how can we tell which are and which aren't?
With some APODs it's pretty clear, like NGC 7331 and Beyond of July 12th, where you have the galaxy group seen beyond what is clearly a screen of relatively foreground stars, but where the picture is imaging stars individually in the cluster it doesn't seem quite so obvious.

http://apod.nasa.gov/apod/ap080803.html

http://apod.nasa.gov/apod/ap050617.html

Simply beautiful; isn't it? I think it is pretty amazing that Hubble can bring in that cluster with such clarity from the SMC. Notice the two globular clusters outside the SMC.
Orin

An open cluster is thought to be formed from a molecular cloud (much like M42), so all stars in the cluster are made up from roughly the same material, thus should have similar chemical composition. This can be determined by spectroscopy.

Stars in open clusters tend to be massive and bright. I would think the brightest in today's picture are prime candidates for inclusion in the cluster.

All the stars in the SMC are about 210,000 light-years away from us, thus the imaged brightness is a good indicator for absolute brightness, as the dimmer stars are not (much) dimmer because of distance.

What puzzles me is the blue and red stars. Shouldn't stars in an open cluster all be the same color? Or are the reds not part of the cluster (there seems to be a lot of them in the image)?

Hello Case,

ICBW (I could/can be wrong!) but I think the red stars are older, cooler stars. I don't know for certain if they are red shifted because they are moving away but It appears they may be too close , perhaps foreground stars, for that to be a factor. JMHO.

Prospero_again wrote:Of the stars we can see are all of them part of the cluster, and if not how can we tell which are and which aren't?

That is hard to tell, since the open cluster is in another galaxy. Like the Pleiades and Hyades in our own galaxy, they can be identified by their motion. Since they form a group of gravitationally bound members (more or less), they all have the same kind of motion, relative to our sun. This motion is harder to identify in todays APOD, since the open cluster is in another galaxy. All stars in that galaxy tend to have the same motion. Since the distance between the stars of the cluster and the 'ordinary' Magelhanic stars is of the same order, the difference is hard to tell without dedicated equipment, like case has written.

An open cluster has about 100 stars, ranging from 10 to 1000. They are clustered -hence the name- and usually gravitationally bound. Since this cluster is young, the stars that transform their hydrogen into helium and beyond in a very fast pace are not yet vanished. That explains the relative high abundance of blue stars in young open clusters. Open clusters do have ordinary red and yellow stars.

Part of the gas and dust clouds might still be present. The stars will look a bit 'wooley'. Note that not all of the wooley appearance is caused by gass and dust clouds, since the high brightness of some stars, outshining the background stars, will contribute as well.

Case wrote:Stars in open clusters tend to be massive and bright. I would think the brightest in today's picture are prime candidates for inclusion in the cluster.

All the stars in the SMC are about 210,000 light-years away from us, thus the imaged brightness is a good indicator for absolute brightness, as the dimmer stars are not (much) dimmer because of distance.

What puzzles me is the blue and red stars. Shouldn't stars in an open cluster all be the same color? Or are the reds not part of the cluster (there seems to be a lot of them in the image)?

Compare NGC 290 with the 30 times closer NGC 869/884 (a.k.a., h and chi Persei):

http://antwrp.gsfc.nasa.gov/apod/ap071207.html
http://antwrp.gsfc.nasa.gov/apod/ap060413.html
--------------------------------------------------------
http://seds.org/messier/Xtra/ngc/n0869.html

<<["Chi" and "h" Persei]'s hottest main sequence stars are of spectral type B0.>>
--------------------------------------------------------
Hence, their brighter "O" stars have evolved into stable red giants
with very few yellow stars in between:

I wonder what the night sky would look like from a planet orbiting one of the stars in the cluster depicted in today's apod.

Incidentally, APOD is my absolute most favorite web site, bar none. The folks who make it possible do a splendid job of keeping it undertandable.

Peter Mockridge wrote:I wonder what the night sky would look like from a planet orbiting one of the stars in the cluster depicted in today's apod.

"...hundreds of [bright] stars and spans about 65 light years across [volume ~250,000 ly^3]"
makes for 1 star per 1,000 light years or an average bright star separation of ~ 10 lys.

The NGC 290 stars are -5 to -6 Absolute magnitude
and the more distant ones (at 10 parsecs/32.6156 lys)
would therefore be -5 to -6 Apparent magnitude or
about the brightness of The Crab Supernova of 1054 AD.

The 8 nearest bright stars at ~ 5*sqrt(3) lys , however,
would be ~13 times [= 2.7 magnitudes]
brighter or ~ -8 Apparent magnitude.
--------------------------------------------
Apparent magnitudes of known celestial objects:

App. Mag. Celestial object

−12.6 Full Moon
−8.0 Maximum brightness of an iridium flare
−6.0 The Crab Supernova (SN 1054) of 1054 AD (6500 light years away)
−4.7 Maximum brightness of Venus and the International Space Station.
−3.9 Faintest objects observable during the day with naked eye
--------------------------------------------

I know very little about the magnitude of stars in our vicinity, so please forgive my ignorance when I ask: is it possible that we are in an open cluster and haven't realized it yet?

Earthbelow wrote:I know very little about the magnitude of stars in our vicinity, so please forgive my ignorance when I ask: is it possible that we are in an open cluster and haven't realized it yet?

_______ No.

However, our Sun is moving through the Local Interstellar Cloud [LIC] that was only recently detected by being "lit up" by Nova Cygni 1992:
http://antwrp.gsfc.nasa.gov/apod/ap050116.html
-----------------------------------------
http://antwrp.gsfc.nasa.gov/apod/ap020210.html

<<The stars are not alone. In the disk of our Milky Way Galaxy about 10 percent of visible matter is in the form of gas, called the interstellar medium (ISM). The ISM is not uniform, and shows patchiness even near our Sun. It can be quite difficult to detect the local ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however, absorbs some very specific colors that can be detected in the light of the nearest stars. A working map of the local ISM within 10 light-years based on recent observations is shown above. These observations show that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region. Our Sun may exit the Local Interstellar Cloud during the next 10,000 years. Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth.>>
-----------------------------------------
http://antwrp.gsfc.nasa.gov/apod/ap020217.html

<<What surrounds the Sun in this neck of the Milky Way Galaxy? Our current best guess is depicted in the above map of the surrounding 1500 light years constructed from various observations and deductions. Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars. The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange. The Gum Nebula, shown in green, is a region of hot ionized hydrogen gas. Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC. Future observations should help astronomers discern more about the local Galactic Neighborhood and how it might have affected Earth's past climate.
-----------------------------------------
The sun may have climate problems passing through the denser Aquila Rift molecular cloud:

http://antwrp.gsfc.nasa.gov/apod/ap990503.html

Thanks Guys. I'm not sure I'm much the wiser about my question, but i enjoyed the discussion.
I did notice that the second link from Orin
http://apod.nasa.gov/apod/ap050617.html
being a wider view did seem to show that we look at the SMC thru a drift of stars from our galaxy, so at least some of those must figure in the close up. Seems that if you knew the relative scales and indeed how sharp the photography in both you could calculate how many foreground stars you would expect across the cluster. But presumably it is more likely to be the apparently brighter ones with the spike effects, and the redder ones?

Prospero_again wrote:Thanks Guys... Seems that if you knew the relative scales and indeed how sharp the photography in both you could calculate how many foreground stars you would expect across the cluster. But presumably it is more likely to be the apparently brighter ones with the spike effects, and the redder ones?

I think you can safely assume all of the spiked stars are local foreground stars, and that the majority of the remainder are from NGC 290 and the SMC.

bystander wrote:

Prospero_again wrote:Thanks Guys... Seems that if you knew the relative scales and indeed how sharp the photography in both you could calculate how many foreground stars you would expect across the cluster. But presumably it is more likely to be the apparently brighter ones with the spike effects, and the redder ones?

I think you can safely assume all of the spiked stars are local foreground stars, and that the majority of the remainder are from NGC 290 and the SMC.

I have to disagree. All of the brightest stars, including small ones, in the photo have spikes. If you remove them all on the assumption that they are foreground stars, you will end up with a picture that doesn't look like an open cluster, just a regular star field. It was a 26 minute exposure, so the bright stars in the cluster would end up with the spikes from the Hubble's secondary mirror supports.

Gary