MargaritaMc wrote:Thank you so much, Anthony and Ann! I'm thoroughly gob-smacked at how much consideration you both gave to answering my question. And, yes, both answers helped me a lot.
Ann, you wrote (about the H-R diagram) "the connection between stellar mass, brightness and temperature." In this connection, does stellar radius (which is marked on the diagram) equate to mass?
Again - a million thanks!
ps. I've checked out NGC 3293 and NGC 3324, (as in Ann's suggestion) and saw the pink nebulosity mentioned in Ann's first post. If I'm not being too ask-ask-asking, what causes that? It is hugely pretty!
Bye the bye, am I the only person who thought that Apod of the Fornax cluster looked like a really up-market jeweller's display? With NGC 1365 being a superb brooch - the sort the Queen (I'm English!) wears on State occasions.
pps. LOVED the bread flour analogy! Really helped me to "get"it.
You're welcome, Margarita, and I'm glad the bread analogy helped!
Stellar radius does equal mass as long as the star is on the main sequence
. The main sequence red dwarfs are little runts, but the massive blue main sequence O-stars are considerably larger than the Sun. Professor emeritus Jim Kaler is the expert I turn to for information about individual stars. You may check out this page
, if you would like to read about a (more or less well-known) star that interests you.
One of my own favorite main sequence O-type stars is 10 Lacertae. Jim Kaler wrote about that star
From a surface heated to a quite-amazing 32,000 Kelvin, it radiates a with a luminosity of 26,800 Suns (the majority of the light in the invisible ultraviolet), from which we derive a radius 4.7 times that of the Sun and a great mass of 16 times solar.
The best-known (and extremely puny) red dwarf is probably Proxima Centauri. Its claim to fame is that it is the closest of all stars from the Earth (with the obvious exception of the Sun). Jim Kaler wrote about Proxima Centauri
As a mid-class M (M5.5) dwarf star, Proxima is faint indeed, to the eye 18,000 times dimmer than the Sun. From Alpha Cen proper, Proxima would appear as only fourth magnitude, about as bright as the faint stars of the Little Dipper appear to us. When infrared radiation produced by its 3040 Kelvin surface is accounted for, it is seen to be more luminous, but still only 1/600 as bright as the Sun (and 15 percent the size), the result of a mass only 12 percent solar.
But when the stars leave the main sequence, there is no longer any connection between size and mass. Check out this (large, 1.7 MB) series of pictures of the sizes of certain planets and stars
The first picture shows the relative sizes of the inner planets of the Solar System, Mercury, Venus, the Earth and Mars. As you can see, Mars is surprisingly puny.
The second picture compares the size of the Earth with the sizes of the ice giants Uranus and Neptune and the sizes of the gas giants Jupiter and Saturn.
The third picture compares the size of Jupiter with the sizes of Wolf 359 (one of those typical tiny red dwarf stars) and with the size of the Sun and A-type main sequence star Sirius. (You shouldn't believe in that horrible orange-yellow color of the Sun in that picture. It's okay to think of the Sun as very
pale yellow - although the Sun really is white - but it absolutely
The fourth picture compares the size of Sirius with the sizes of Pollux, Arcturus and Aldebaran. The latter three stars are all red giants, which have given up hydrogen fusion in their cores. Pollux is the most modest of the three, the smallest, the faintest and "the least cool". Jim Kaler wrote about Pollux
(which is one of the two luminous stars in the constellation Gemini):
Jim Kaler wrote about Aldebaran
From its distance of 34 light years, we calculate a total luminosity (incuding infrared radiation) for Pollux 46 times that of the Sun, and coupled with its temperature (4770 Kelvin), a diameter some 10 times solar, making it smaller than most of its cool giant brethren and only a quarter the dimension of Aldebaran.
From luminosity and temperature, the mass comes in at around 1.8 times solar.
Aldebaran's surface temperature of 4010 degrees Kelvin (compared to the Sun's 5780 degree temperature) gives it a distinct orangy color not all that dissimilar to that of Mars, which commonly passes it. Allowance for infrared radiation reveals the star to have a fairly high luminosity 425 times that of the Sun, which leads to a radius of 43 times solar.
From the theory of stellar structure and evolution, Aldebaran carries a mass of around 1.7 times that of the Sun.
As you can see, Jim Kaler says that Pollux is probably more
massive than Aldebaran, even though Aldebaran is much larger and about ten times brighter. These two stars are at different stages in their evolution as red giants. The Sun, too, will become a red giant one day, and it will certainly reach a stage in its evolution when it is brighter and larger than Pollux is now.
The fifth picture compares Aldebaran with Rigel, Antares and Betelgeuse. Jim Kaler wrote about Rigel
Jim Kaler wrote about Betelgeuse
At a distance of 860 light years (second Hipparcos reduction), it shines with the light of 85,000 Suns after account is taken of ultraviolet light from its 11,500 Kelvin surface. The two combine to tell of a radius swollen to 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury. Direct measure of angular diameter leads to a similar radius of 73 times solar, showing that the star's various properties are accurate. The theory of stellar structure and evolution shows that the star must carry a mass of close 18 times that of the Sun, and indicates that it has a dead helium core and is still in a swelling and cooling phase.
At a compromise distance of 570 light years, and allowing for a lot of infrared radiation and for absorption of light by circumstellar dust, the luminosity comes in at 85,000 times that of the Sun, considerably more than comes out of Antares. At the larger distance, luminosity boosts up to 105,000 Suns. From these and the temperature, we derive respective radii of 3.1 and 3.4 Astronomical Units, more than double the size of the Martian orbit.
An astronomical unit is ≈ 200 times the radius of the Sun. So if the size of Betelgeuse is more than three astronomical units, then the radius of Betelgeuse is more than 600 times the radius of the Sun.
Jim Kaler wrote about Betelgeuse
We do not really know the star's condition at the moment, but the odds are that it is now in the process of fusing helium into carbon and oxygen in its core. From theory, its initial mass should have fallen somewhere around 18 or 19 times that of the Sun.
As you can see, the two supergiants Rigel and Betelgeuse are thought to be quite similarly bright and similarly massive, although they are at different stages of their evolution as supergiants and therefore vastly different in size and surface temperature.
The sixth picture shows some of the largest red supergiants known. Jim Kaler wrote about Mu Cephei and VV Cephei:
Mu Cep has a current estimated radius somewhere between 1200 and 1650 times that of the Sun, or 5.6 to 7.7 Astronomical Units, bigger than the orbit of Jupiter. Though VV may well top it out, the uncertainties preclude accurate assessment.
Radius and temperature give a luminosity that falls somewhere between 275,000 and 575,000 times that of the Sun, which in turn give masses between 25 and 40 times solar.
As for why there is pink nebulosity around blue star cluster NGC 3293, it's because there is hydrogen gas not far from the cluster, and the combined ultraviolet emission from the hot stars knocks electrons around the nucleus of a hydrogen atom (and a hydrogen atom only has one proton and one electron) into "a higher orbit". In that higher orbit, the electrons have more energy than they do in their "ground state". But soon the electrons "fall back" to their ground state again, and as they do so, they radiate their extra energy away. Most of the electrons radiate a deep red hydrogen alpha photon, but some radiate a blue-green hydrogen beta photon. The combined light is pink.