Starship Asterisk*

Interesting, Geck. It makes sense that many, if not most, lenticular galaxies must retain some structure from their dusty and starforming days.

Of the galaxies you picked, NGC 5308 is redder than NGC 4762, while NGC 4111 is very similar to NGC 4762 in color. The obvious dust structure in NGC 4111 might that there is a non-negligent "moderately young" to intermediate population in that galaxy, which would make the overall color bluer. But since the U-B color is relatively red, such a population can't be all that young and ultraviolet.

Ann

From looking through Hubble's archive, I've found lenticulars to be interestingly varied in both structure and dust content. They invariably present with symmetry and sometimes some faint dust bits but the structures are also often barely discernible. I can make up for that with a couple of high pass filters in Photoshop. For instance, the core of NGC 4026 which you've just mentioned:
[attachment=0]hst_09107_a1_wfpc2_f555w_pc_sci.jpg[/attachment]

Of course, it's really only possible to see these using Hubble or some other advanced telescope. Otherwise, lenticulars are indeed very plain looking.


Another example, NGC 5308:
[url=https://flic.kr/p/pLCWwS][img]https://farm4.staticflickr.com/3933/15601338966_11804c966d.jpg[/img][/url]

NGC 4111. Try not to let the yellow color get to you on this one. It probably doesn't mean as much as you might think. If you fancy yourself a keen observer, see if you can spot the slight dimming within the white part of the galactic disc. Think of it as something like Saturn's E ring viewed nearly edgewise. Of course, the conspicuous ring of dust encircling the nucleus at a perpendicular angle is especially noteworthy for this one.
[url=https://flic.kr/p/oChPvo][img]https://farm6.staticflickr.com/5585/14850524356_42e42ba2b4.jpg[/img][/url]

As I leafed through James D Wray's [i]The Color Atlas of Galaxies[/i], I came across this beautiful specimen of a lenticular galaxy, [url=http://astrim.free.fr/NGC4026.htm]NGC 4026[/url]. The discription in the link calls NGC 4026 an edge-on spiral galaxy, but that's not what it is. In James D Wray's closeup of the galaxy, taken with the McDonald 2.1 meter telescope, we can see that there is no dust in the galaxy and no structure at all, apart from the very prominent disk. James D. Wray wrote:

[quote]This is the type example of an S0 galaxy (disk, no evident dust and no recent star formation.)[/quote]

As I checked up NGC 4026, I found that it was small, smaller indeed than NGC 4762. Yes, but it is noticeably redder than NGC 4762. It is farther away and therefore likely more affected by intergalactic dust, but on the other hand it is located in the direction of Ursa Major, where there is comparatively little dust between ourselves and other galaxies.

The only lenticular galaxy I could find that is comparative in color to NGC 4762 is [url=http://www.noao.edu/outreach/aop/observers/n205.html]NGC 205[/url], the larger but fainter of the Andromeda galaxy's two obvious satellite galaxies. The B-V colors of NGC 205 and NGC 4762 are similar, but the U-B color of NGC 205 is noticeably bluer than the U-B color of NGC 4762. And indeed, Adam Block's image of NGC 205 shows the existence of not only two small dust clouds in Andromeda's satellite galaxy, but also the presence of a small bluish population near the center of this galaxy. So it could be that NGC 205 should not actually be classified as an S0 galaxy, since it does contain dust and even some moderately recent star formation.

I think I stand corrected when I said that small galaxies aren't very red. That can't be true, since I found some quite red and quite small S0 galaxies. But at least I was correct when I said that NGC 4762 is indeed a very white S0-type galaxy.

Ann

[quote="BDanielMayfield"][quote="Chris Peterson"]If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.[/quote]
I infer by that that lenticulars are rare then. What percentage of galaxies are classed as lenticular?[/quote]

I don't think they are very rare at all. When you look at a dense cluster of galaxies, where all the (centrally located) galaxies are obviously composed of old red stars only, several of these galaxies may well be lenticulars.

I made a quick check in James D Wray's [i]The Color Atlas of Galaxies[/i]. Of course the selection of galaxies that you find in an atlas may not be very representative of the true population of galaxies out there. There are, however, many lenticulars in Wray's atlas. As I checked up two rather red ones, NGC 1533 and NGC 1543, I found that they were quite small. Ooops! :oops:

But in any case, I can easily imagine that spirals may evolve into lenticulars, but I don't think it necessarily follows that lenticulars evolve into ellipticals.

Ann

[quote="BDanielMayfield"][quote="Chris Peterson"]If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.[/quote]
I infer by that that lenticulars are rare then. What percentage of galaxies are classed as lenticular?[/quote]
No idea. My observation was based on the fact that both lenticulars and ellipticals are comprised of similar populations of old stars. If the lenticular phase were a long one ending in ellipticals, I'd expect different stellar ages.

[quote="Chris Peterson"]If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.[/quote]
I infer by that that lenticulars are rare then. What percentage of galaxies are classed as lenticular?

Oh. Wikipedia makes it sound so certain...

[quote="geckzilla"]Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral.[/quote]
That's true morphologically, but there's not much consensus that it's true in terms of galactic evolution. Old galaxies (ellipticals and lenticulars) have almost all their stellar metrics in common. If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.

[quote]Geck wrote:
Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral. If ellipticals are older, it would figure that they are generally redder.[/quote]

I'm not sure that ellipticals are generally redder than lenticulars. I feel certain that very large galaxies with no star formation are generally redder than small galaxies with no star formation. Large galaxies are typically very metal-rich, and if the largest ellipticals are larger than the largest lenticulars, they will probably also be redder in color. [url=http://www.csmonitor.com/Science/2010/1201/Universe-might-hold-three-times-more-stars-than-previously-thought]Recently, it was suggested that giant elliptical galaxies contain so many small red dwarf stars that their abundance triples the the number of stars in the universe[/url].

The reason why giant ellipticals contain so many red dwarfs is most likely that they have undergone so very many episodes of star formation, because every bout of star formation gives rise to a large number of tiny and incredibly long-lived red dwarfs. But every new generation of stars is typically, on average, more metal-rich and therefore somewhat redder than previous generations.

But as for the color of ellipticals versus lenticulars, please note that NGC 4754, the apparent neighbour of NGC 4762, is classified as a lenticular galaxy (SB0), and it is yellower than NGC 4762. Of course, I believe that NGC 4754 is also more distant than NGC 4762 and therefore more reddened by intergalactic dust. But if it is more distant, it is also larger than NGC 4762, and size, as I said, is a factor that often contributes to "redness".

Ann

Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral. If ellipticals are older, it would figure that they are generally redder. Of course, when processing an image, a good goal to have is to assume something in the picture is white. It helps to balance the colors and is very generally correct but by no means perfect.

Well, the fact that the galaxy looks extremely white in the Hubble photo doesn't necessarily mean much at all, because it may simply have been processed to look that way. But it [i]is[/i] true that the color doesn't seem to vary at all, so the mix of stellar populations seems to be the same everywhere. There are no concentrations of particularly yellow stars anywhere, and no concentrations of blue stars anywhere.

But also the galaxy's so called B-V index (which is what astronomers use to measure the color of stars and galaxies) is relatively white, and definitely whiter than I would expect for a lenticular galaxy (that is, a galaxy with a disk but no gas or dust, like NGC 4762).

Finally, as you could perhaps see, NGC 4762 looks definitely whiter than its apparent neighbour NGC 4754 in Josef Pöppel and Beate Behle's image (the one I posted a link to).

So all in all, NGC 4762 strikes me as interestingly white. Well spotted, John! :D

Ann

Gosh! The simian eye is as accurate as a telescope!

Please explain, Anne. When you say it is anomolously white, is that based on observations or on that image, which I suspect has been PhotoShopped to make it very white?

John
PS I imply no scientific skullduggery, just a wish for the APOD to impress.

Good question, Bruce. I'd say that any galaxy, old or young, must contain red giants, and in a galaxy where all the stars are old the brightest stars will be red giants. It seems to me that it should be possible to resolve a sufficient number of red giants in NGC 4762 in order to make a decent estimate about the distance to this galaxy, because red giants can be standard candles too. They typically reach a maximum brightness before they turn into white dwarfs - these are the brightest medium-mass M giants. But the red giants also reach another, lower, maximum brightness as they exhaust their supply of helium in their cores and "clump" together at the top of the red horizontal giant branch - these are the red clump stars of bright spectral class K.

If we could find a way to resolve a sufficient number of red giants, we should be able to say how far away NGC 4762 is. But I must say that the stellar population of this galaxy seems to be amazingly well mixed.

Ann

You make a good case for NGC 4762 being both closer and smaller than advertised Ann, but how can uncertainty about such a galaxy's distance be resolved? I guess we would need a standard candle Supernova to pop off there before we could tell for sure.

I have to return to the color of NGC 4762. It really [i]is[/i] very white. [url=http://www.capella-observatory.com/ImageHTMLs/Galaxies/NGC4754_62.htm]In this image by Josef Pöpsel and Beate Behle[/url] it looks definitely whiter than its apparent neighbour, NGC 4754. NGC 4762 is so white, indeed, that if we bear in mind that it must be somewhat reddened by interstellar dust along our line of sight, it hardly makes sense for it to be a large Virgo Cluster galaxy that has undergone many episodes of star formation and built up a large population of metal-rich stars. But if it is a relatively small galaxy, one that has undergone one or two violent bursts of star formation whose subsequent fireworks of supernovas blew all its gas and dust away, then it makes sense for this galaxy to have a metal-poor population that is large enough to affect the overall color of it. More precisely, it makes sense for it not to have built up a completely dominant metal-rich population that totally swamps the light of the metal-poor stars in it.

But if this is a smallish galaxy, then it can't belong to the Virgo Cluster proper. At the very least, it must be much closer to us than the typical Virgo Cluster galaxies. And indeed, according Principal Galaxy Catalog, the redshift and recession velocity of NGC 4762 suggests that it is only about 32 million light-years away, compared with about 56 million light-years for its apparent neighbour, and obvious Virgo Cluster member, NGC 4754. Admittedly the Virgo Cluster is so nearby that the proper motion of a galaxy in it may cancel out, or at least strongly counteract, the cosmological redshift of it.

But if NGC 4762 is a smallish foreground galaxy, then it makes sense for it to be whiter than NGC 4754, which certainly belongs to the Virgo Cluster proper.

I have to wonder if NGC 4762 can be compared with an older version of [url=http://www.nature.com/nature/journal/v473/n7346/images/473160a-f1.2.jpg]M82[/url], the nearby starburst galaxy. M 82 is already losing copious amounts of gas and dust due to the strong winds of its brilliant OB stars and the violence of its many supernovas. Is it possible that M82 will one day end up looking like NGC 4762 does today?

Ann

[quote="Chris Peterson"][quote="Guest"]My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow![/quote]
The glow is just stars. You're not seeing scattered or re-emitted light.[/quote]

With all that out there, for all to see.... there must be life...

My first thought on seeing today's APOD that it was a pulsar.

[attachment=1]2235003118.jpg[/attachment]

I suppose, should the day come that we could see one from a relatively close distance, it really wouldn't look this way?? More like below - but closer?

[url]http://apod.nasa.gov/apod/ap000609.html[/url]

Probably be more like looking at a light-house beacon. More apt to see it if it was pointing at us.

[attachment=0]NEWPORT SUNSET.JPG[/attachment]

The sideways beams would need something off which to reflect? If so, that would be quite a sight too!!

[quote="APOD Robot"]is so thin that it is actually [url=http://adsabs.harvard.edu/abs/1989ApJ...339..783H]difficult to determine[/url] what type of disk galaxy it is[/quote]
There is actually a category of galaxy called "superthin"! :D

[quote="Ann"]It's the other way round, Chris. The larger the B-V index, the yellower is the galaxy.[/quote]
Of course. I was reading large B as brighter. Duh. Shouldn't post before breakfast.

[quote="Chris Peterson"][quote="Ann"]Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun. [/quote]
That isn't apparent to me. Its b-v is greater than that of the Sun. I'd say that makes it less yellow (although not by a lot).[/quote]

It's the other way round, Chris. The larger the B-V index, the yellower is the galaxy. Anyway, this is a typical old population galaxy, where most of the light comes from stars that are either the color of the Sun, or yellower than the Sun.

It should be noted that a galaxy like this one is nothing like a globular cluster, whose B-V indexes may be comparatively low, in spite of the great age of the constituent stars. That is because the globulars typically contain a moderately large population of metal-poor blue horizontal branch stars. But a large galaxy like NGC 4762 is mostly made up of metal-rich stars, so the fraction of blue horizontal stars is very low.

Ann

[quote="Ann"]Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun. [/quote]
That isn't apparent to me. Its b-v is greater than that of the Sun. I'd say that makes it less yellow (although not by a lot).

[quote]JohnD wrote:
That's a very white galaxy![/quote]

Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun. [url=http://1.bp.blogspot.com/-OIGs88kieEA/T_Xu4OS8s_I/AAAAAAAAQMk/XMU6i_huAFs/s1600/challenge+103+-+hot%252C+hot%252C+hot+-++include+a+sun.jpg]If you believe that the Sun is yellow,[/url] then you should remember that this galaxy is yellower than the Sun.

Ann

[quote="Guest"]My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow![/quote]
The glow is just stars. You're not seeing scattered or re-emitted light.

My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow!

i wonder if there is a thin mint galaxy :P

That's a very pretty picture. I do like it. Interesting to see the galaxy at that angle.