Starship Asterisk*

NGC 602 and Beyond

Explanation: Near the outskirts of the Small Magellanic Cloud, a satellite galaxy some 200 thousand light-years distant, lies 5 million year young star cluster NGC 602. Surrounded by natal gas and dust, NGC 602 is featured in this stunning Hubble image of the region, augmented by images in the X-ray by Chandra, and in the infrared by Spitzer. Fantastic ridges and swept back shapes strongly suggest that energetic radiation and shock waves from NGC 602's massive young stars have eroded the dusty material and triggered a [url=http://heritage.stsci.edu/2007/04/supplemental.html" >progression of star formation moving away from the cluster's center. At the estimated distance of the Small Magellanic Cloud, <a href="http://chandra.harvard.edu/photo/2013/ngc602/]the Picture[/url] spans about 200 light-years, but a tantalizing assortment of background galaxies are also visible in this sharp multi-colored view. The background galaxies are hundreds of millions of light-years or more beyond NGC 602.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

I'm just saying: JAWS!!!!
But seriously, this is a fantastic picture! Let's just look at the optical part of it and consider the fantastic central star cluster itself, NGC 602. This cluster is a brilliant illustration of the Initial Mass Function, which says that in a given burst of star formation, mother nature will always make more low-mass than high-mass stars. Look at the central cluster which contains - how many high-mass stars? Seven? Twelve? - how there is an absolute swarm of low-mass stars buzzing about the high-mass ones like bees.

And look at the star with the bow shock at center left! Could it be a runaway star from the bright elongated cluster at far left?

Also look at the splendid spiral galaxy at left which can be seen so clearly right through this mayhem of star formation!




And now look at the full-wavelength image at right to see what Spitzer and Chandra can reveal to us. Look at the brilliantly red glow with some white inside it to the lower left of of the main cluster. The white things - surely they must be stars? - can be seen in the optical image as well. Surely this must be one - or more likely two - young clusters in an earlier phase of star formation?

And look at the brilliant purple splotch to the left of the main cluster. It seems to correspond to an insignificant star in the optical image. What is it? To be as bright in X-rays as this, aren't we talking about an accretion disk? Could it possibly be a stellar-mass black hole stealing matter from a nearby companion?

And look! There is a faint purple halo centered on a small elongated galaxy at lower left! Is than an X-ray-emitting galaxy with jets? Or is it a galaxy with a hot X-ray halo? Probably the latter.

And look at those two small purple dots embedded in the wall of a prominent ridge. Aren't they very young stars in the process of forming, at the stage when baby stars typically emit X-ray?

There are some other purple splotches that make me wonder as well. What are they? Distant galaxies with active galactic nuclei? Magnetars inside the Small Magellanic cloud itself? Something else entirely?

Wow again! There is so much to see and wonder about in this image!

Ann

Incredible....well done.

:---[===] *

This is a fantastic photo. It's a classroom for a semester intro astronomy class all by itself. Thanks, Teacher Ann!

So are the majority of the stars in the center part of the surrounding nebula, then? Obviously, the galaxy to the left wouldn't be, but those bright stars in the midst of the "jaws" (vivid image!). If so, then, I'd imagine that this is kind of what the Pleiades might have looked like a few thousand years ago (or more).

Ann wrote:... This cluster is a brilliant illustration of the Initial Mass Function, which says that in a given burst of star formation, mother nature will always make more low-mass than high-mass stars. Look at the central cluster which contains - how many high-mass stars? Seven? Twelve? - how there is an absolute swarm of low-mass stars buzzing about the high-mass ones like bees. ...
Ann

Ann, thanks for your survey of some of the wonders in this image.

The excerpt above brings me to a very very basic astronomy question. I have excerpted the image as well, to part of the region you discussed. My first reaction, intuitively from non-astronomical experience, is that the two largest round white discs in the image are "big" stars, and then all of the smaller points of white, some of them still of appreciable size in number of pixels, but then on down to many that look like they're only one or two pixels, is that these are smaller stars, or perhaps stars that are farther away, so they're dimmer or smaller.

But I learned some time ago (from this forum) some facts that change this view. The first thing I learned is that really, if you work it out, Hubble's camera has a certain granularity, as does any camera, and its pixel size is such that every star in this image is so far away that every star in this image, in truth, should occupy less than a pixel, even the biggest, brightest, and closest of them. (Today, I finally did this as a sort of a homework problem: The largest star known,Y Scuti, has a radius of 2.4E+9 km. The Small Magellanic cloud is estimated at 2.0E+5 light years = 1.88E+18km, so that star, at that distance, would subtend a visual angle of -- rounding up -- about one ten-millionth of a degree. I don't know the pixel size on Hubbel's camera, but it can't be anywhere near that small.)

Nevertheless, some of these stars are hitting Hubbel's camera with more photons/time than others. So, there ought to be some way to represent a brighter point light source, compared to a less bright point light source. But how does this work? Does the light bleed into neighboring pixels according to some law? For the larger stars, you can also see a blue aura around them in the image, and of course the diffraction spikes. I could give an explanation of the diffraction spikes. I'm not sure how correct my understanding is. No idea about the blue auras. What causes these?

But back to my main question -- how I should understand and interpret when one star looks bigger than another in a Hubbel image, what is really going on?

Thanks for the kind words, NCTOM and Mark!

E Fish, I was thinking of the Pleiades too, just like you. But the brightest and most massive star in the Pleiades, Alcyone, is a slightly evolved 6 solar mass star of spectral class B7, and while such a star would be utterly overwhelming and deadly if it were to replace the Sun in the solar system, it is modest compared with the brightest stars in NGC 602. In NGC 602, we can be sure that there ar several O-type stars of at least 15 solar masses each, and in fact 15 solar masses would be the barest minimum for an O-type star.

Wikipedia wrote:
NGC 602 includes many young O and B stars and young stellar objects, with few evolved stars.[9] Ionisation in the nebula is dominated by Sk 183, an extremely hot O3 main sequence star visible as the bright isolated star at the centre of the Hubble image.

A star of spectral class O3 can be expected to contain at least 60 solar masses, and 80 solar masses, or even 100, seem like clear possibilities.

What I'm saying is that the Pleiades is nothing like NGC 602. But let's not forget that the Pleiades is at least 100 million years old, while NGC 602 is perhaps 5 million! What kind of stars did the Pleiades contain when it was 5 million years old? In my opinion, the Pleiades is such an impressive cluster that it ought to have contained an O-star or two in its earliest youth, but they must have exploded as supernovas long ago.

Ann

MarkBour wrote:
how I should understand and interpret when one star looks bigger than another in a Hubble image, what is really going on?

A bright star produces a saturated Airy disk
whose wings fall off as the cube of the radius (= 1/r³).

Hence, if bright star C is 8 times brighter than bright star D
then star C should have 2 times the diameter of star D.

(However, all dim stars will be the same size.)

https://en.wikipedia.org/wiki/Airy_disk wrote:
<<The Airy disk (or Airy disc) and Airy pattern are descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light.>>

MarkBour wrote:

Ann wrote:... This cluster is a brilliant illustration of the Initial Mass Function, which says that in a given burst of star formation, mother nature will always make more low-mass than high-mass stars. Look at the central cluster which contains - how many high-mass stars? Seven? Twelve? - how there is an absolute swarm of low-mass stars buzzing about the high-mass ones like bees. ...
Ann

Ann, thanks for your survey of some of the wonders in this image.

The excerpt above brings me to a very very basic astronomy question. I have excerpted the image as well, to part of the region you discussed.

Capture.png

My first reaction, intuitively from non-astronomical experience, is that the two largest round white discs in the image are "big" stars, and then all of the smaller points of white, some of them still of appreciable size in number of pixels, but then on down to many that look like they're only one or two pixels, is that these are smaller stars, or perhaps stars that are farther away, so they're dimmer or smaller.

But I learned some time ago (from this forum) some facts that change this view. The first thing I learned is that really, if you work it out, Hubble's camera has a certain granularity, as does any camera, and its pixel size is such that every star in this image is so far away that every star in this image, in truth, should occupy less than a pixel, even the biggest, brightest, and closest of them. (Today, I finally did this as a sort of a homework problem: The largest star known,Y Scuti, has a radius of 2.4E+9 km. The Small Magellanic cloud is estimated at 2.0E+5 light years = 1.88E+18km, so that star, at that distance, would subtend a visual angle of -- rounding up -- about one ten-millionth of a degree. I don't know the pixel size on Hubbel's camera, but it can't be anywhere near that small.)

Nevertheless, some of these stars are hitting Hubbel's camera with more photons/time than others. So, there ought to be some way to represent a brighter point light source, compared to a less bright point light source. But how does this work? Does the light bleed into neighboring pixels according to some law?

I am the wrong person to answer that question, but yes - pixel bleeding is the answer I would give you.

For the larger stars, you can also see a blue aura around them in the image, and of course the diffraction spikes. I could give an explanation of the diffraction spikes. I'm not sure how correct my understanding is. No idea about the blue auras. What causes these?

A possibility is that the larger stars are simply bluer than the smaller ones. But I would be careful about reading too much into the colors of a Hubble image. The filters used for the Hubble image of NGC 602 were F555W (V), F658N (H-alpha+[N II]), and F814W (I). F555W is a wideband filter centered on yellow-green light. F658N is a narrowband filter centered on red Hα light, the typical red light of emission nebulas, and N II, which is almost the same color and wavelength as Hα, but which represents a lower level of ionization. An F658N filter can be thought of as a "nebula filter". F814W, finally, is an infrared filter, which explains why we can see some infrared details in the Hubble optical image.

But it is important to understand that the Hubble image is not a "true-color" RGB image. An extremely good RGB picture can reveal subtle differences in the colors of stars, like Roberto Mura's brilliant image of IC 2602. There are no diffraction spikes in that image. Don't ask me why.

But back to my main question -- how I should understand and interpret when one star looks bigger than another in a Hubbel image, what is really going on?

Bright stars cause pixel bleeding. That is the only answer I can offer.

Oh, I can see that Art has already given you a perfect answer to your question!

Ann

Ann wrote:Bright stars cause pixel bleeding. That is the only answer I can offer.

Oh, I can see that Art has already given you a perfect answer to your question! :D

There is a concept of "pixel bleeding". It occurs with all astronomical CCDs, and is called blooming. It is caused by electrons spilling out of a fully filled pixel and into adjacent pixels in a column. We don't see it in this image, either because it has been manually edited out during processing, or because the brightest pixels are not saturated. Blooming is almost never removed from scientific images, but is usually removed from images intended for aesthetic purposes.



Broadened stars with space telescopes are caused by two things: one is diffraction, as Art previously explained. The other is scatter from optical elements, such as the primary mirror (which is not perfectly smooth) and the surfaces of various filters. The scatter is apparent in the radial streaks around the bright stars (not to be confused with the diffraction spikes).

With ground-based telescopes, we don't normally see the diffraction pattern, because atmospheric seeing spreads out bright star images so they are larger than the Airy disk. Of course, that's not an issue in space.