Starship Asterisk*

BillT wrote: ↑Fri Jun 21, 2019 12:42 am

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.

The lack of spikes is because the image was taken using a refractor, which one would presume has a "perfectly" circular aperture. Although you can see what appears to be multiple faint rays. Often DSLR lenses have an eight bladed aperture stop, which creates interesting starburst diffraction pattern if the lens is wound down from fully open : https://www.slrlounge.com/diffraction-a ... t-effects/. So I'm not sure what would be causing the faint rays unless it is some diffraction caused by the camera itself or maybe the telescope baffles.

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.

Ann wrote: ↑Thu Jun 20, 2019 8:18 am

AVAO wrote: ↑Thu Jun 20, 2019 7:43 am

Ann wrote: ↑Thu Jun 20, 2019 7:10 am

The "blurrier" the picture is, the harder it is to tell the difference between stars and galaxies. In today's APOD there are no long spikes emanating from the stars, which makes it even harder to tell them from elliptical or face-on spiral galaxies. Instead of spikes, there is a multitude of little "rays" surrounding the bright stars.

So in a picture like this, you can really only pick out the galaxies by their elongated shapes or by the fluff surrounding them, or by their low surface brightness.

In highly resolved Hubble images, it is usually easy as pie to spot the galaxies. You've got to be careful though, because a very red small point source could possibly be a highly reddened but intrinsically very bright elliptical galaxy. However, the brightest elliptical galaxies rarely come alone, so they should normally be surrounded by other little reddened galaxies close to them. A small red-but-bright-in-the-middle point source that is all alone is probably a star.

Why don't you check out the Hubble picture of the Tadpole galaxy, where there is a bonanza of background galaxies to be enjoyed?

Ann

Ann you're right.

The hubble picture shows the difference relatively well in the highest resolution under http://hubblesite.org/image/3144/news.

Source: NASA, Hubble

Source: These images were taken by a combination of the Spitzer (infrared), Chandra (x-ray), European Space Agency’s XMM-Newton (x-ray), and Hubble (optical) space telescopes, and the National Radio Astronomy Observatory’s Very Long Baseline Array, and the Very Large Array (radio) in New Mexico.

I was asking myself, if it would not be possible to compare the x-ray with the optical one. As far as I know, there are relatively few stars in the x-ray range but most of the galaxies?

Jack from the AVAO Team

Click to view full size image

The Milky Way in X-rays. Source:
https://apod.nasa.gov/apod/ap960102.html

As you can see from the very old all-sky picture at left, X-rays are typically emitted by a small number of discrete sources.

Few stars emit X-rays. Pulsars and some neutron stars do, as well as a small number of "extreme binaries", where colliding winds at extreme speeds and temperatures generate X-rays.

Amm

AVAO wrote: ↑Thu Jun 20, 2019 7:43 am

Ann wrote: ↑Thu Jun 20, 2019 7:10 am

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.

The "blurrier" the picture is, the harder it is to tell the difference between stars and galaxies. In today's APOD there are no long spikes emanating from the stars, which makes it even harder to tell them from elliptical or face-on spiral galaxies. Instead of spikes, there is a multitude of little "rays" surrounding the bright stars.

So in a picture like this, you can really only pick out the galaxies by their elongated shapes or by the fluff surrounding them, or by their low surface brightness.

In highly resolved Hubble images, it is usually easy as pie to spot the galaxies. You've got to be careful though, because a very red small point source could possibly be a highly reddened but intrinsically very bright elliptical galaxy. However, the brightest elliptical galaxies rarely come alone, so they should normally be surrounded by other little reddened galaxies close to them. A small red-but-bright-in-the-middle point source that is all alone is probably a star.

Why don't you check out the Hubble picture of the Tadpole galaxy, where there is a bonanza of background galaxies to be enjoyed?

Ann

Ann you're right.

The hubble picture shows the difference relatively well in the highest resolution under http://hubblesite.org/image/3144/news.

Click to view full size image

Source: NASA, Hubble

Click to view full size image

Source: These images were taken by a combination of the Spitzer (infrared), Chandra (x-ray), European Space Agency’s XMM-Newton (x-ray), and Hubble (optical) space telescopes, and the National Radio Astronomy Observatory’s Very Long Baseline Array, and the Very Large Array (radio) in New Mexico.

I was asking myself, if it would not be possible to compare the x-ray with the optical one. As far as I know, there are relatively few stars in the x-ray range but most of the galaxies?

Jack from the AVAO Team

Ann wrote: ↑Thu Jun 20, 2019 7:10 am

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.

The "blurrier" the picture is, the harder it is to tell the difference between stars and galaxies. In today's APOD there are no long spikes emanating from the stars, which makes it even harder to tell them from elliptical or face-on spiral galaxies. Instead of spikes, there is a multitude of little "rays" surrounding the bright stars.

So in a picture like this, you can really only pick out the galaxies by their elongated shapes or by the fluff surrounding them, or by their low surface brightness.

In highly resolved Hubble images, it is usually easy as pie to spot the galaxies. You've got to be careful though, because a very red small point source could possibly be a highly reddened but intrinsically very bright elliptical galaxy. However, the brightest elliptical galaxies rarely come alone, so they should normally be surrounded by other little reddened galaxies close to them. A small red-but-bright-in-the-middle point source that is all alone is probably a star.

Why don't you check out the Hubble picture of the Tadpole galaxy, where there is a bonanza of background galaxies to be enjoyed?

Ann

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.

shaileshs wrote: ↑Thu Jun 20, 2019 4:48 am Sorry if I've already asked this in the past. Whenever I look at such photos, I always wonder, which one of the bright objects (of various sizes, shapes and colors) we see are stars and which ones are galaxies. There are some photos wherein every object (almost every) is a galaxy. But, some have a mix of both.. It becomes especially tricky when stars don't show spikes and when some galaxies can be round shaped.. Is there a easy/quick way of knowing ? Thanks in advance for all answers/comments.