Starship Asterisk*

A View Toward M106

Explanation: Big, bright, beautiful spiral, Messier 106 dominates this cosmic vista. The nearly two degree wide telescopic field of view looks toward the well-trained constellation Canes Venatici, near the handle of the Big Dipper. Also known as NGC 4258, M106 is about 80,000 light-years across and 23.5 million light-years away, the largest member of the Canes II galaxy group. For a far away galaxy, the distance to M106 is well-known in part because it can be directly measured by tracking this galaxy's remarkable maser, or microwave laser emission. Very rare but naturally occurring, the maser emission is produced by water molecules in molecular clouds orbiting its active galactic nucleus. Another prominent spiral galaxy on the scene, viewed nearly edge-on, is NGC 4217 below and right of M106. The distance to NGC 4217 is much less well-known, estimated to be about 60 million light-years.

<< Previous APOD

This Day in APOD

Next APOD >>

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.

Very good image...I find it amazing... Maser...with water... AMAZING...so far away...I don't equate Galaxy with Water... I mean...how much is there that it can DO this???

Amazing...
:---[===] *

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.

Ummm...at the risk of being wrong...in THIS particular image at least I think... anything very round and a tad fuzzy, but solid looking is a star... anything elongated, etc...a galaxy... you can click on the image and get a big picture that you can click and magnify to see better.
I hope that helps...

:---[===] *

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 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.

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

There is one more thing to consider when it comes to telling the stars from the galaxies in astroimages. According to the caption of today's APOD, M106 is only 23.5 million light-years away, and its surface brightness is going to be comparatively high. The Tadpole galaxy, by contrast, is some 420 million light-years away, almost 18 times as far away as M106, and it is some six magnitudes fainter than M106 in the sky.

Because the Tadpole galaxy is relatively faint "compared with its surroundings" and really extended, sharp-eyed Hubble had to photograph a lot of sky around it and caught an impressive haul of background galaxies to boot. But because M106 is relatively bright in the sky compared with its surroundings, and because today's APOD was taken by an amateur with a small ground-based telescope, only a relatively small number of background galaxies will be bright enough to really give away their "galactic nature" in a picture like today's APOD.

Many of the foreground stars in today's APOD are bright enough to be part of the Tycho star catalogue. Some have even had their color indexes measured. There are five NGC and one UGC galaxy in the M106 picture apart from M106 itself. You can bet your boots that none of the foreground stars in the Hubble Tadpole galaxy image can be found in the Tycho catalogue, and none of the background galaxies have earned their own catalogue entries.

Ann

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

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.

Ann

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

ThanX Ann.

I made a quick check. The star in the yellow ring has no X-ray correspondence (in red). So the method could work...

Great Galaxy!

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.

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.

All of the apertures in the imaging system are circular. The rays are caused by scatter from imperfect optical surfaces, not diffraction at all.

Spiral galaxies remind me of elliptical galaxies with spiral arms.. .