R Monocerotis, the star illuminating Hubble's Variable Nebula, is a pre-main sequence star. It is not a fully formed star yet, it is still accreting mass, it shines because it is still shrinking, and it has not switched on its core hydrogen fusion yet.
Note that the stars that have not reached the main sequence yet are usually larger and also brighter than they will be when they first reach the main sequence. The main sequence is the curved gray line in the illustration.
Blue giants are typically hot stars that have used up the hydrogen in their cores and expanded due to shrinking, but they have not expanded all that much. They have expanded some, but not by an enormous amount. To give you an idea of the sizes of blue giants, take a look at the sizes of the most important stars of the Pleiades versus the size of the Sun:
The Sun is the small orange star at far left. The color of the Sun is wrong, but never mind. The star closest to the Sun is Pleione, which is spectral class B8V. Pleione is still fusing hydrogen to helium in its core, just like the Sun is doing.
Yes, but take a look at all the other blue stars, which are all somewhat evolved and somewhat "swollen". Look at the star that is second from the right, which is Atlas. The spectral class of this star is B8III, exactly the same as the classification of R Mon of Hubble's Variable Nebula. Yes, but Atlas is an evolved star, which means that it has expanded because it has used up the hydrogen in its core, but R Mon is a pre-main sequence star that has not even started fusing the hydrogen in its center yet! Ah yes, but these two stars may be the same size! Atlas is inflating and R Mon is shrinking!
Astronomers can tell that a star is a giant because its spectral lines are narrower (and, I think, sometimes not as deep):
Let's get back to what Wikipedia said about R Mon, the illuminating star of Hubble's Variable Nebula. According to the "column of facts" at right on that page, the mass of R Mon is ~2–10 M☉, or 2-10 solar masses. Come on! That's a huge span, and a star of 2 solar masses will be so different from a star of 10 solar masses! If the mass of a star is 2 solar masses, the star will be a Sirius and live a mostly quiet life for several hundred million years before turning into first a red giant and then a white dwarf.
But if R Mon is 10 solar masses, it will likely go supernova in way less than a hundred million years:
Seeing R Mon go supernova would have been fun! But we will really have to find the fountain of youth if we are going to be ringside spectators of this cosmic spectacle. Assuming that R Mon really is massive enough to go supernova in the first place, of course.
And finally the variable nebula. It is believed to be caused by fast-moving clouds rather deep inside the nebula, casting "variable shadows ":
Ann
Note that the stars that have not reached the main sequence yet are usually larger and also brighter than they will be when they first reach the main sequence. The main sequence is the curved gray line in the illustration.
I've got to stop here and ask you to look at the spectral classification of R Mon, B8IIIe. I don't know about the "e", but the "III" means, in star speak, a star that is brighter than a main sequence star because it is a "regular giant".Wikipedia wrote about R Monocerotis, the star illuminating Hubble's Variable Nebula:
R Monocerotis, abbreviated R Mon, is a very young binary star system in the equatorial constellation of Monoceros. The apparent magnitude of R Mon varies between 10 and 12 and the spectral type is B8IIIe.
Wikipedia wrote:
Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main-sequence stars, class sd (or VI) for subdwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.
Blue giants are typically hot stars that have used up the hydrogen in their cores and expanded due to shrinking, but they have not expanded all that much. They have expanded some, but not by an enormous amount. To give you an idea of the sizes of blue giants, take a look at the sizes of the most important stars of the Pleiades versus the size of the Sun:
The Sun is the small orange star at far left. The color of the Sun is wrong, but never mind. The star closest to the Sun is Pleione, which is spectral class B8V. Pleione is still fusing hydrogen to helium in its core, just like the Sun is doing.
Yes, but take a look at all the other blue stars, which are all somewhat evolved and somewhat "swollen". Look at the star that is second from the right, which is Atlas. The spectral class of this star is B8III, exactly the same as the classification of R Mon of Hubble's Variable Nebula. Yes, but Atlas is an evolved star, which means that it has expanded because it has used up the hydrogen in its core, but R Mon is a pre-main sequence star that has not even started fusing the hydrogen in its center yet! Ah yes, but these two stars may be the same size! Atlas is inflating and R Mon is shrinking!
Astronomers can tell that a star is a giant because its spectral lines are narrower (and, I think, sometimes not as deep):
Spectral lines of three A-type stars. The green one has narrow lines and is a giant.
Credit: Michael Richmond
Credit: Michael Richmond
Let's get back to what Wikipedia said about R Mon, the illuminating star of Hubble's Variable Nebula. According to the "column of facts" at right on that page, the mass of R Mon is ~2–10 M☉, or 2-10 solar masses. Come on! That's a huge span, and a star of 2 solar masses will be so different from a star of 10 solar masses! If the mass of a star is 2 solar masses, the star will be a Sirius and live a mostly quiet life for several hundred million years before turning into first a red giant and then a white dwarf.
But if R Mon is 10 solar masses, it will likely go supernova in way less than a hundred million years:
Click to play embedded YouTube video.
Seeing R Mon go supernova would have been fun! But we will really have to find the fountain of youth if we are going to be ringside spectators of this cosmic spectacle. Assuming that R Mon really is massive enough to go supernova in the first place, of course.
And finally the variable nebula. It is believed to be caused by fast-moving clouds rather deep inside the nebula, casting "variable shadows ":
Ann
Statistics: Posted by Ann — Fri Jan 31, 2025 6:14 pm — Replies 3 — Views 136