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parallaxicality
2018-Dec-10, 11:23 PM
Can't believe it took me this long to look at it (http://sci.esa.int/science-e-media/img/26/ESA_Gaia_DR2_HRD_Gaia_625.jpg)

At this resolution, all kinds of things are suddenly noticeable; for instance, there appear to be a number of "on ramps" onto the main sequence, which suggests there are different ways for stars to form, as well as at least one other "off ramp" onto the giant branch, suggesting there are different ways for stars to die. But what really interests me is that bug cloud over at the middle far left. I've never seen that population before. Could it be something new?

StupendousMan
2018-Dec-11, 12:52 AM
If you refer to the small clump of stars around absolute magnitude G = 4-5, and temperature around 30,000 Kelvin, my first guess was that they might represent the blue end of the horizontal branch (metal-poor stars which fuse helium to hydrogen in their cores). However, when I visited

https://www.cosmos.esa.int/web/gaia/gaiadr2_hrd

I saw that this feature is strongest among stars which are members of the thin disk -- relatively metal-rich, and relatively young. So, my revised guess is that these are the central stars of planetary nebulae, which have lost their outer layers in a series of very gentle puffs. The very hot, small, cores will gradually slide down and slightly to the right on the diagram to join the white dwarfs.

Good question. There are a LOT of features in these diagrams, aren't there?

Hornblower
2018-Dec-15, 01:40 AM
Can't believe it took me this long to look at it (http://sci.esa.int/science-e-media/img/26/ESA_Gaia_DR2_HRD_Gaia_625.jpg)

At this resolution, all kinds of things are suddenly noticeable; for instance, there appear to be a number of "on ramps" onto the main sequence, which suggests there are different ways for stars to form, as well as at least one other "off ramp" onto the giant branch, suggesting there are different ways for stars to die. But what really interests me is that bug cloud over at the middle far left. I've never seen that population before. Could it be something new?

Please tell us which features you are describing as on ramps and off ramps. I don't see anything for which such a description is meaningful.

parallaxicality
2018-Dec-15, 09:24 AM
Never tried an attachment before; let's see if it works/

Hornblower
2018-Dec-15, 02:05 PM
Never tried an attachment before; let's see if it works/

Your arrows appear to be pointing to some clumps of higher abundance of stars in some parts of the distribution. There may be astrophysical reasons for them rather than mere selection effects, but I would not interpret them as junctions with on ramps or off ramps. See this set of images of HR diagrams, some of which illustrate evolutionary tracks:

https://www.google.com/search?q=stellar+evolution+diagram&rlz=1C1CHBF_enUS781US781&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwjexuba4aDfAhVIx1kKHa4cBBEQsAR6BAgAEAE#i mgrc=8p2HqtNkdQ3SXM:

Some of them show the tracks of contracting protostars while others show the tracks as the stars evolve off the main sequence and become giants. The protostar tracks could be envisioned as on ramps while the giant tracks could be off ramps. In both cases there appears to be a continuous distribution of entrance and exit points over the full length of the main sequence, with the position depending on the mass of the star.

selden
2018-Dec-16, 02:04 PM
Unfortunately, those Googled HR diagrams are pre-Gaia, which are based on poorer statistics than Gaia DR2 has provided.

There seems to be a consensus that many of the subtle features seen in Gaia's HR diagrams are real. Some features become more prominent when one eliminates stars with large errors in their measurements, and those with low apparent magnitudes. This effect is discussed in the Gaia publication "Gaia Data Release 2: Observational Hertzsprung-Russell diagrams" which is available at https://arxiv.org/abs/1804.09378

In particular, one of the "on ramp" features that parallaxicality mentioned is included in that paper's Figure 8. It seems to be the tail of a broadening of the main sequence which is is explained as being caused by unresolved binaries with identical components. I've attached a screengrab of that figure.

Some other examples of the power of Gaia's large sample when analyzing its HR diagram are
+ the hydrogen/helium bifurcation of the white dwarf region of the HR diagram, which is shown at http://sci.esa.int/gaia/60209-white-dwarfs-in-gaia-s-hertzsprung-russell-diagram/
+ the effects of the differing metallicity of various globular clusters, shown at https://www.gaia.ac.uk/multimedia/gaia-dr2-hr-diagram-globular-clusters
+ a very narrow narrow gap has been found in the HR diagram of M-class stars. See https://aasnova.org/2018/07/11/3796/ and https://arxiv.org/pdf/1806.07792.pdf