Starship Asterisk*

As you may or may not know, Gaia is a cutting-edge space satellite for astrometry, which means that its primary job is to determine the distances to a billion stars in the Milky Way. Check out the Gaia homepage to learn more!

Last Thursday, one of the most important movers and shakers behind the entire Gaia project, Lennart Lindegren, visited our local astronomy club, ASTB, and gave an informal speech on the science of Gaia. He spent some time talking about a discovery he almost made, but missed, because he used too large pixels in his depiction of the main sequence. A very narrow gap in the lower main sequence!

Wei-Chun Jao et al wrote:

We present the discovery of a gap near MG ≈10 in the main sequence on the Hertzsprung-Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with MG representing luminosity and GBP − GRP representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the Gaia photometry — it also appears when using near-IR photometry (J − Ks vs MKs). The gap is very narrow (∼0.05 mag) and is near the luminosity-temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the H-R Diagram that hints at an underlying astrophysical cause and we propose that it is linked to the onset of full convection in M dwarfs.

The narrow gap in the lower main sequence may have to do with how heat is transported inside these small stars. All main sequence stars produce their energy at their very centers. Then this energy is transported to the surface of the star by the means of either radiation or convection, or both. Think of convection as "boiling". It is a slower means of transmitting energy than radiation.

Below a mass of 0.35 M_⊕, a star is fully convective. All the energy is transported from the center to the surface of the star through the means of convection. But above 0.35 M_⊕, a small radiative zone is formed at the very center of the star. Here energy is transported more efficiently, which leads to a tiny, tiny jump in luminosity. Hence the gap in luminosity between stars above and below the mass of 0.35 M_⊕!






Another truly fascinating tidbit was that the main sequences of open clusters typically show a second main sequence located above the "normal" one. The stars in the second main sequence are the same temperature as the stars in the "normal" main sequence, but twice as bright. These are unresolved binaries!












Lennart Lindegren didn't actually talk about globular clusters, but I can't resist showing you this picture from the arxiv paper by Wei-Jun Jao et al. It shows how metallicity affects the colors of the stars in globular clusters. Metal-poor clusters have blue horizontal branches and only moderately red RGB and AGB branches. Metal-rich clusters, by contrast, have short red horizontal branches and very red RGB and AGB branches. What a difference metallicity makes!

Finally, the chairman of our astronomy club, who is obsessed with finding life in space, asked Lennart Lindegren if Gaia can't be used to find Dyson spheres in space. These would be tremendously large structures made by super-advanced civilizations to trap and make use of the energy produced by their sun. Couldn't Gaia spot these humongous constructs?


Lennart Lindegren answered that Gaia has looked at millions of stars already, and trying to find Dyson spheres around some of them based on their spectrum alone would be like looking for a needle in a haystack. Well, objected our chairman, couldn't Gaia look more closely at the stars whose spectrum is weird?

Every tenth star we look at is weird, said Lindegren. Every tenth star - imagine!

There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy. Hamlet (1.5.167-8).

Ann

Ann wrote: ↑Sat Sep 29, 2018 6:47 am
Below a mass of 0.35 M_⊕, a star is fully convective. All the energy is transported from the center to the surface of the star through the means of convection. But above 0.35 M_⊕, a small radiative zone is formed at the very center of the star. Here energy is transported more efficiently, which leads to a tiny, tiny jump in luminosity. Hence the gap in luminosity between stars above and below the mass of 0.35 M_⊕!>>

Fascinating stuff, Ann; however, you probably meant to write M_☉ rather than M_⊕:

https://en.wikipedia.org/wiki/Solar_mass wrote:
<<The solar mass (M_☉) is a standard unit of mass in astronomy. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies. It is equal to the mass of the Sun (denoted by the solar symbol ⊙︎). This equates to about two nonillion (two quintillion in the long scale) kilograms:

M_☉ = (1.98847±0.00007)×10³⁰ kg

The above mass is about 332,946 times the mass of Earth (M_⊕), or 1,048 times the mass of Jupiter (M_J).>>

Ann wrote: ↑Sat Sep 29, 2018 6:47 am
Every tenth star we look at is weird, said Lindegren. Every tenth star - imagine!

There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy. Hamlet (1.5.167-8).

https://www.etymonline.com/word/weird#etymonline_v_4898 wrote:
weird (adj.) c. 1400, "having power to control fate, from wierd (n.), from Old English wyrd "fate, chance, fortune; destiny; the Fates," literally "that which comes," from Proto-Germanic *wurthiz (source also of Old Saxon wurd, Old High German wurt "fate," Old Norse urðr "fate, one of the three Norns"), from PIE *wert- "to turn, to wind," (source also of German werden, Old English weorðan "to become"), from root *wer- (2) "to turn, bend." For sense development from "turning" to "becoming," compare phrase turn into "become."

The sense "uncanny, supernatural" developed from Middle English use of weird sisters for the three fates or Norns (in Germanic mythology), the goddesses who controlled human destiny. They were portrayed as odd or frightening in appearance, as in "Macbeth" (and especially in 18th and 19th century productions of it), which led to the adjectival meaning "odd-looking, uncanny" (1815); "odd, strange, disturbingly different" (1820).

• --------------------------------------------------------
  Wm Shaxpere & Anna *WHATEley* of *TEMPLE GRAFTON*
  ...........................................
  <<There is an old English word *WHATE* , meaning fortune, *FATE* , or destiny,
  I think that in a desperate moment of inspiration, confused before the clerk,
  Shakespeare reached into his heart and came out with the name of that Anne
  who would have been his choice, his *FATE* , his destiny.>>
  
  . - _The Late Mr. Shakespeare_ by Robert Nye
  --------------------------------------------------------

https://en.wikipedia.org/wiki/Anne_Hathaway_(wife_of_Shakespeare) wrote:
<<Anne Hathaway (1556 – 6 August 1623) was the wife of William Shakespeare. They were married in 1582, when he was 18 and she was 26 years old. In her father's will, her name is listed as "Agnes", leading to some scholars believing that she should be referred to as "Agnes Hathaway".

Documents from the Episcopal Register at Worcester record in Latin the issuing of a wedding licence to "Wm Shaxpere" and one "Annam WHATEley" of Temple Grafton. The following day, Fulk Sandells and John Richardson, friends of the Hathaway family from Stratford, signed a surety of £40 as a financial guarantee for the wedding of "William Shagspere and Anne Hathwey". Frank Harris, in The Man Shakespeare (1909), argued that these documents are evidence that Shakespeare was involved with two women. He had chosen to marry one, Anne WHATEley, but when this became known he was immediately forced by Hathaway's family to marry their pregnant relative. Harris believed that "Shakespeare's loathing for his wife was measureless" because of his entrapment by her and that this was the spur to his decision to leave Stratford and pursue a career in the theatre. However, most modern scholars take the view that the name WHATEley was "almost certainly the result of clerical error".>>

neufer wrote: ↑Sat Sep 29, 2018 6:51 pm

Ann wrote: ↑Sat Sep 29, 2018 6:47 am
Below a mass of 0.35 M_⊕, a star is fully convective. All the energy is transported from the center to the surface of the star through the means of convection. But above 0.35 M_⊕, a small radiative zone is formed at the very center of the star. Here energy is transported more efficiently, which leads to a tiny, tiny jump in luminosity. Hence the gap in luminosity between stars above and below the mass of 0.35 M_⊕!>>

Fascinating stuff, Ann; however, you probably meant to write M_☉ rather than M_⊕:

https://en.wikipedia.org/wiki/Solar_mass wrote:
<<The solar mass (M_☉) is a standard unit of mass in astronomy. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies. It is equal to the mass of the Sun (denoted by the solar symbol ⊙︎). This equates to about two nonillion (two quintillion in the long scale) kilograms:

M_☉ = (1.98847±0.00007)×10³⁰ kg

The above mass is about 332,946 times the mass of Earth (M_⊕), or 1,048 times the mass of Jupiter (M_J).>>

I did!!!

Ann

There sure seems to be something similar going on in the Hipparcos Catalogue around absolute magnitude +10:

https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram wrote:

---

An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V).

neufer wrote: ↑Sun Sep 30, 2018 2:14 am There sure seems to be something similar going on in the Hipparcos Catalogue around absolute magnitude +10:

https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram wrote:

---

An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V).

You are right. It is visible.

Another point that Lennart Lindegren just briefly touched on, but which is interesting in itself, is the bifurcation of the white dwarf cooling track, which is visible in the Hipparcos Hertzsprung-Russel diagram that you included in your post. Gaia had to rely on SDSS, with its superior photometry, to explain it.

C. Babusiaux et al wrote:

In particular, the u -band fluxes of H-rich DA white dwarfs are suppressed by the Balmer jump at 364.6 nm, which reddens the colours of these stars.

The Balmer jump is in the wavelength range where the Gaia filters calibrated for DR2 differ most from the nominal filters (Evans et al. 2018), which explains the importance of using tracks that are updated to the DR2 filters for the white dwarf studies instead of the nominal tracks provided by Carrasco et al. (2014).

So the Balmer series and the Balmer jump at 364.6 nm explain the bifurcation of the hydrogen and helium white dwarf cooling tracks.

Ann

The gap in the lower main sequence has already been reported by bystander in the Breaking News forum: viewtopic.php?p=284022#p284022

Susanna Kohler of AAS Nova wrote:

In particular, Jao and collaborators suggest that the gap may be related to a known transition in mid-M dwarfs, from larger stars that are mostly convective with a thin radiative layer, to smaller stars that are fully convective.

This suggests that the gap in the lower main sequence is due to a tiny jump in the size of the star as a part of its interior becomes radiative.

Ann

Ann wrote: ↑Sun Sep 30, 2018 10:29 am
The gap in the lower main sequence has already been reported by bystander in the Breaking News forum: viewtopic.php?p=284022#p284022

Susanna Kohler of AAS Nova wrote:

In particular, Jao and collaborators suggest that the gap may be related to a known transition in mid-M dwarfs, from larger stars that are mostly convective with a thin radiative layer, to smaller stars that are fully convective.

This suggests that the gap in the lower main sequence is due to a tiny jump in the size of the star as a part of its interior becomes radiative.

• 1) The interior of ALL stars cool by radiation (& conduction).
  
  2) However, at some stage star cores can no longer cool by convection
  ...thereby forcing them to be hotter and thus 'burn' fuel faster.
  
  3) A stable star that 'burns' fuel faster must be brighter (i.e., jump vertically in the HR diagram).
  
  4) Whether or not there is a simultaneous change in surface temperature (i.e., a horizontal jump in the HR diagram) is unclear.