From Wikipedia page on hypergiants – click on my name for the link :

“The word “hypergiant” is commonly used as a loose term for the most massive stars found, even though there are more precise definitions. In 1956, the astronomers Feast and Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude greater than MV = -7. In 1971, Keenan suggested that the term would only be used for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today. [1] This means that a hypergiant doesn’t necessarily have to be more massive than a similar supergiant. Still, the most massive stars are considered to be hypergiants, and can have masses ranging up to 100-150 solar masses.”

The hypergiants wiki-page also contains a link to the first ever generation of stars(population III) from where :

“Population III or metal-free stars (they contained metals at the very end of their lifetimes – they are said to be metal-free because the metals exist in the core and are unobservable) are a hypothetical population of extremely massive and hot stars with virtually no metal content, except for a small quantity of metals formed in the Big Bang, such as Lithium-7. These stars are believed to have been formed in the early universe. They have not yet been observed directly, but indirect evidence for their existence has been found in a gravitationally lensed galaxy in the very distant universe.[6] They are also thought to be components of faint blue galaxies. Their existence is necessary to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra, as well as the existence of faint blue galaxies.[7] It is believed that these stars triggered a period of reionization.
Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements from the Big Bang, it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred solar masses, which is much larger than the current stars. Analysis of data on low-metallicity Population II stars, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 10 to 100 solar masses instead. This also explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories awaits the launch of NASA’s James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Population III stars.”

If anyone’s interested. I know this is now a very old thread but thought, hey, why not add this info anyway!

2. What about stars in other galaxies perhaps also far less metal-rich and thus more like the population III first stellar generation -could there be O2, O1 & even O0 type stars in such galaxies?

Also the formatting looks odd for some reason. Sigh.

Editing capability here please Bad Astronomer.
..Please, please, please!!!

“… I think the most massive stars in our galaxy are actually O3 – I don’t know that we’ve ever found stars of O0 at all …”

Actually spectroscope, I think Tom Marking got it right first time – there are such things as O0 stars. Any O type hypergiant would be an O0 star -the O being the spectral class & the zero being the designation for hypergiant status (one better than bright supergiant which is Ia.)

That would apply whether the O type hypergiant’s decimalised sub-spectral type (if that’s the term) was 000 ie. O type & hypergiant and the brightest O star decimalised, O01 O07 or OO09.5! Although technically there may need to be spaces there somewhere or slightly different ordering – eg. O0 O,
O1 0, O7 0 etc ..

I think I did see the idea somewhere that the hottest most massive main
sequence or “dwarf” type stars known in our Galaxy were O3 V stars (V =
dwarf / main-sequence*) but then I wonder :

1. What about the very first generation of supermassive stars that were apparently many times larger and more massive, hotter and brighter again than today’s “metals” (elements above Helium) contaminated stars? The super
metal-poor, pure Hydrogen & helium Population III behemoths???

2. What about stars in other galaxies perhaps also far less metal-rich and thus more like the population III first stellar generation -could there be O2, O1 & even O0 in such galaxies?

&

3. What about the most massive hypergiants in our own galaxy today – did they perhaps begin their brilliant careers as O0 V stars? Eg. Eta Carinae (100 Plus solar masses) the Pistol Star (ditto) Plasketts Stars – were they once those very earliest and shortest lived O type ultimates before they quickly exhausted their core hydrogen?

—-

* For those here that don’t already know the luminosity classes convention in astronomy is :

0 = hypergiant, I = Supergiant (split into Ia –bright supergiant, Ib less bright supergiant & even Iab =intermediate supergiant), II =Bright giant, III = giant,
IV= subgiant, V dwarf or main-sequence, VI = Sub-dwarf (metal poor stars) & I think also VII =white dwarf

—-

“…about 40 supernovae are exploding somewhere in the universe every second. However, light from most of these events won’t reach Earth for billions of years , if ever.”
- Page 73, “Ask Astro” in ‘Astronomy ‘magazine October 2008.

The blue supergiant star Rigel A emits more light in a minute than the Sun does in a month and is the most radiant star within a 1,000 light year radius from the Sun.
- Ken Croswell, “The Blue Witch” in ‘Sky &Telescope’ magazine May-June 2007.

“The Ramans do everything in threes.”
- Arthur C. Clarke, ‘Rendezvous with Rama’, Final page (252), Pan
Books Ltd, 1973.

—-

NB. Spell-checked in word & no typos seen – at least as of now.

NGC 4921:
distance = 320 million light-years
apparent magnitude = +13.04
apparent dimensions = 2.5′ x 2.2′
real dimensions = 233,000 x 205,000 light-years

M51 (Whirlpool galaxy):
distance = 23 million light-years
apparent magnitude = +9.0
apparent dimensions = 11.2′ x 6.9′
real dimensions = 75,000 x 46,000 light-years

If M51 was located 320 million light-years away it would be 194 times dimmer than it is. It would have an apparent magnitude of +14.72 which is 1.68 magnitudes dimmer than NGC 4921 (i.e., 4.69 times dimmer than NGC 4921). But M51 has 0.072 times the area of NGC 4921 so we would expect it to be 13.8 times dimmer than NGC 4921 assuming the two have the same luminosity per unit area. So the luminosity per unit area for M51 is 2.95 times what it is for NGC 4921.

Based on that number we can estimate when NGC 4921 had its gas removed. Let’s assume that the most luminous main sequence stars in M51 are spectral class O3V: luminosity = 16,300 suns, mass = 16.0 solar masses, lifetime = 10 million years. The most luminous main sequence stars in NGC 4921 should have a mass of 9.3 solar masses: spectral class = B1V: luminosity = 2,070 suns, lifetime = 43 million years.

So NGC 4921 had its gas removed a relatively short time ago (~40 million years) in terms of galactic time scales. This result has some uncertainty due to the use of visual only magnitudes for the two galaxies in question instead of the total multi-band luminosity.

Having such a recent loss of its gas points to some recent catastrophic event in the history of the galaxy. If the cause was the interaction with the extragalactic medium then one would have expected NGC 4921 to have lost its gas billions of years ago. So I’m a bit suspicious of that explanation.

“I still think the Andromeda Galaxy is the most beautiful ever. Love seeing competitors.”

Well my choice for the winner of the “most beautiful galaxy ever award” goes to spiral galaxy NGC 2997 in Antlia.

M51, the whirlpool galaxy isn’t too far behind it & then in third place overall but first visually – as seen with the unaided eye rather than photos or telescope – its got to be the Large Magellanic Cloud!

Perhaps the BA could set up a vote and post images of some likely contenders on this blog? One for the suggestion box Phil Plait!

quasidog said on Feb 6th, 2009 at 5:18 pm
@ ChazInMT

“I have one theory here …. coincidence. The object right in the center giving off those ’star’ like rays (which I believe are caused by diffraction of light off the secondary mirror arms) ….is actually a star. It could just be that a star just happens to be right in the center of the shot, between Hubble and the galaxy itself. If that is the case its an amazing alignment.”

One way to test this idea would be to use a spectroscope & see what the spectrum revals whether its a particular star type for a single or binary star or a blend of a whole lot of them indicating an AGN – wouldn’t that work? Has that been done? Anyone know?

@ Bein’Silly : I agree it looks kinda milk-white to me, like frothed egg white or something in colour? Then again, it is a false colour image.

Regardless of what the brightest stars that have actually been found in the Milky Way galaxy the principle is the same.

“problem two: Stars of class AO V (main-sequence dwarf) may be the most massive but would they be brightest or would there be some orange or red giant stars that are brighter despite being less massive because of their greater surface area?”

The luminosity function L = M^3.5 only applies to main sequence stars. However, the overwhelming majority of stars are on the main sequence. Also, the red giants that evolve from more massive stars tend to be more luminous than the red giants that evolve from less massive stars. Thus, from the standpoint of comparing the overall luminosities of two galaxies the comparison between red giants in one galaxy versus those in the other galaxy, is roughly similar to the comparison between the main sequence stars.

“problem three: Is this taking into account evolving sub-dwarf (metal-poor) stars?”

Unless this is a very ancient galaxy whose stars were born before there was much enrichment of the interstellar medium with metals there should be relatively few of these stars.

Based on this blog posting I would imagine that NGC4921 must be a low luminosity galaxy. Any numbers for what its overall luminosity is?

Depending on when NGC4921 got its gas removed it will have less massive stars than the Milky Way. For example, if NGC4921 got its gas removed 600 million years ago then the most massive stars it has would be spectral class A0V with a mass of 3.04 solar masses (they would just now be nearing the end of their lifetime). Thus, the luminosity would be 3% of the Milky Way’s luminosity (or about 3.0E35 watts).

Ok problem one:

I think the most massive stars inour galaxy areactually O3 – Idon’t know that we’ve ever found stars of O0 at all …

& problem two:

Stars of class AO V (main-sequence dwarf) may be the most massive but would they be brightest or would there be some orange or red giant stars that are brighter despite being less massive because of their greater surface area?

& problem three

Is this taking into account evolving sub-dwarf (metal-poor) stars?

Not meaning to knock you, Tom Marking – just some hopefully constructive feedback to consider.

The luminosity of each star goes as: L(M) = M^3.5

But low-luminosity stars are much more frequent than high-luminosity stars:
N(M) = k * M^(-2.5)

The total luminosity of the galaxy should be something like:

L-galaxy = integral of L(M) * N(M) * dM

= integral of M^3.5 * k * M^(-2.5) * dM

= integral of k * M * dM

= 0.5*k * (M-max^2 – M-min^2)

So the total luminosity of the galaxy should be proportional to the square of the maximum mass of stars contained in it.

A galaxy like our Milky Way has spectral type O0 main sequence stars with a mass of 16.8 solar masses. Of course these stars only last 8.6 million years.

Depending on when NGC4921 got its gas removed it will have less massive stars than the Milky Way. For example, if NGC4921 got its gas removed 600 million years ago then the most massive stars it has would be spectral class A0V with a mass of 3.04 solar masses (they would just now be nearing the end of their lifetime). Thus, the luminosity would be 3% of the Milky Way’s luminosity (or about 3.0E35 watts).

The original gas inside the galaxy is gravitationally bound to it. It therefore must take energy to dislodge it. Are there any research reports containing what the density of the gas outside the galaxy is compared to the density of the gas inside it? Also, what is the velocity of NGC4921 compared to the outside gas? I would imagine that there must have been a strong shockwave produced by the interaction of the incoming gas and the native gas in the galaxy.

{Checks screen, zooms up again, cuts & pastes}

“But the galaxy looks blue! Yes, in real life this galaxy should look reddish, but this image is false-color: it was taken using a yellow filter and a near-infrared one. So what you see as bluish in the image is actually yellow or red to the eye. What’s seen as red is actually infrared. Weird, isn’t it?”

Yup. He said ‘blue.’

Which is indeed very weird because it looks very white to me.

Perhaps I need my eyes checked but that galaxy NCG whatever it was looks like cappuccino froth or that white icing on a doughnut as I see it.

Hmm … I’m feeling hungry now!
Its a sign from the heavens – remember extra cream!

Quickly checks again .. before heading to Doughnut King. Yes, looks definightly milky and not-so much blue. Am I wrong people?

Still blue or white, its an awesome image BA.
THX for sharing it with us.

See the annotated image at http://www.spacetelescope.org/images/html/heic0901b.html

I have one theory here …. coincidence. The object right in the center giving off those ‘star’ like rays (which I believe are caused by diffraction of light off the secondary mirror arms) ….is actually a star.

It could just be that a star just happens to be right in the center of the shot, between Hubble and the galaxy itself. If that is the case its an amazing alignment.

Might be wrong, it could just be an extremely active galactic nuclei but my guess is that there is a a very bright start in between.

Just a guess.

The Local Group (our cluster) is really the only naked eye cluster you can see … there are 35 galaxies in all but a lot of them to the naked eye are barely visible anyway. There are some cool little maps on the following wiki link that give an idea of what is there, mainly little satellite galaxies.

http://en.wikipedia.org/wiki/Local_Group

On the next link’ there is a listing for naked eye groups a bit down the page. Thought I would just post these for reference anyway.

http://en.wikipedia.org/wiki/List_of_galaxy_clusters

There are so many awesome galaxies to see in that picture. At first I thought it was because they were in the same cluster, meaning that if one were to live on a planet orbiting a star somewhere nice in one of the galaxies in the cluster, I would see a gazillion galaxies with their spiral arms blazing and elliptical discs spread out in their fuzzy glory.

But that’s not really the case – those spiral galaxies are *behind* the cluster in which the main galaxy in the image resides, according to Phil. This makes sense – when taking pictures at “normal” scales with, say, a telephoto lens, the background is comparatively less small than what you’re focusing on, than if you were right up at your object, because the field of view is so narrow. Much more so, of course, on the scale of hundreds of thousands of light-years.

So my question is: is there a place in the Universe where I could crane my neck, look up, and see a bunch of galaxies as the colorful swirls that we love so much? In other words – how dense are galaxy clusters? The Milky Way is a free-floating galaxy, right? Except for our little dance with Andromeda, of course.

Hopefully we are the only planet in the Universe with short-sighted beings.

And it ain’t a galaxy, it’s the Omega Point as seen from deep space! See how all the other galaxies are being pulled into it!

Wonder what it would look like out “there” if we could see all the Dark matter/energy in EM wavelengths???

Now, if we could just get funding for a really BIG space telescope, like say, 200 meters in diameter???

Only 8 million such photos to encapsulate the entire visible universe? I predict, if such is ever accomplished, that someone will see a conglomeration of such objects forming a picture of God,,,or maybe just Jerry Garcia,,,

Gary 7

But along with the above photo and some of Andromeda I think the Sombrero is right up there. I rarely stare at astronomy photos but I did that with the Hi-Res Sombrero ones for about 15 minutes (which is an epic long time for an ADD head like me).

@Sci-Fi- The scale to me is somehow humbling but sort of inspiring at the same time. That doesn’t make any sense, it’s weird that I get that feeling from that photo. But it’s the way I felt with the Deep Field shots too. Awesome.