Vacuum Telescopes

The light from a celestial object is bounced and distorted as it penetrates the Earth’s turbulent atmosphere, and this image degradation continues all the way into the telescope.  Currents of air within the telescope tube caused by parts of the tube or optics being at different temperatures can severely degrade a telescope image, particularly in a large telescope.

Nowhere is this more apparent than in a professional solar telescope.  Sunlight entering the telescope heats up the inside of the telescope and optical components, resulting in turbulent air currents that make the images less sharp than they could be.

To solve this problem, some solar telescopes contain a vacuum so there is no air to heat and therefore no image distortion within the telescope.  This requires, however, a rather thick piece of glass (of high optical quality, of course) at the front of the telescope in order to maintain the vacuum within the tube.  A good example of this kind of telescope is the Swedish 1-m Solar Telescope (SST) located on the island of La Palma in the Canary Islands.

A much thinner front lens can be used if the telescope tube is filled with helium rather than evacuated, and though the results are much better than an air-filled telescope tube, they are not quite as good as with a vacuum telescope.

I am not aware of any vacuum telescopes being used for nighttime observations.

Two Paths to Low Mass

A brown dwarf (also known as an infrared dwarf) is, in a way, a failed star.  Early in their lives, these ultra-low-mass stars (13+ MJ) fuse deuterium into helium-3, and in the highest mass brown dwarfs (65-80 MJ) lithium is depleted into helium-4, as shown below.

But the mass is too low for fusion to be sustained (the temperature and pressure in the core aren’t high enough), and soon the fusion reactions peter out.  Then, only the slow process of thermal contraction provides a source of heat for the wanna-be star.

There is another, very different, path to a brown dwarf star.  A cataclysmic variable usually consists of a white dwarf and a normal star in a close binary system.  As material is pulled off the “donor star” (as the normal star is called) onto the white dwarf, the donor star can eventually lose so much mass that it can no longer sustain fusion in its core, and it becomes a brown dwarf star.

When we see a white dwarf / brown dwarf binary system, how do we know that the brown dwarf wasn’t always a brown dwarf?  Strong X-ray and ultraviolet emission provides evidence of an accretion disk around the white dwarf, and astronomers can calculate the rate of mass transfer between the two stars.  Often, this is billions of tons per second!  Using other techniques to estimate the age of the binary system, we sometimes find that the donor star must have started out as a normal star with much more mass than we see today.

Metallicity

No, it’s not the name of a rock band. Astronomers (unlike everybody else) consider all elements besides hydrogen and helium to be metals. For example, our Sun has a metallicity of at least 2% by mass (Vagnozzi 2016). That means as much as 98% of the mass of the Sun is hydrogen (~73%) and helium (~25%), with 2% being everything else.

Traditionally, elemental abundances in the Sun have been measured using spectroscopy of the Sun’s photosphere.  In principle, stronger spectral lines (usually absorption) of an element indicate a greater abundance of that element, but deriving the correct proportions from the cacophony of spectral lines is challenging.

A more direct approach to measuring the Sun’s elemental abundances is analyzing the composition of the solar wind, though the material blown away from the surface of the Sun that we measure near Earth’s orbit may be somewhat different from the actual photospheric composition.  The solar wind appears to best reflect the composition of the Sun’s photosphere in the solar polar regions near solar minimum.  The Ulysses spacecraft made solar wind measurements above both the Sun’s north and south polar regions during the 1994-1995 solar minimum.  Analysis of these Ulysses data indicate the most abundant elements are (after hydrogen and helium, in order of abundance): oxygen, carbon, nitrogen, magnesium, silicon, neon, iron, and sulfur—though one analysis of the data shows that neon is the third most abundant element (after carbon).

The elephant in the room is, of course, are the photospheric abundances we measure using spectroscopy or the collection of solar wind particles indicative of the Sun’s composition as a whole?  As it turns out, we do have ways to probe the interior of the Sun.  Both helioseismology and the flux of neutrinos emanating from the Sun are sensitive to metal abundances within the Sun.  Helioseismology is the study of the propagation of acoustic pressure waves (p-waves) within the Sun.  Neutrino flux is devilishly hard to measure since neutrinos so seldom interact with the matter in our instruments.  Our studies of the interior of the Sun (except for sophisticated computer models) are still in their infancy.

You might imagine that if measuring the metallicity of the Sun in our own front yard is this difficult, then measuring it for other stars presents an even more formidable challenge.

In practice, metallicity is usually expressed as the abundance of iron relative to hydrogen.  Even though iron is only the seventh most abundant metal (in the Sun, at least), it has 26 electrons, leading to the formation of many spectral lines corresponding to the various ionization states within a wide range of temperature and pressure regimes.  Of the metals having a higher abundance than iron, silicon has the largest number of electrons, only 14, and it does not form nearly as many spectral lines in the visible part of the spectrum as does iron.  Thus defined, the metallicity of the Sun [Fe/H] = 0.00 by definition.  It is a logarithmic scale: [Fe/H] = -1.0 indicates an abundance of iron relative to hydrogen just 1/10 that of the Sun.  [Fe/H] = +1.0 indicates an abundance of iron relative to hydrogen 10 times that of the Sun.

The relationship between stellar metallicity and the existence and nature of exoplanets is an active topic of research.  It is complicated by the fact that we can never say for certain that a star does not have planets, since our observational techniques are strongly biased towards detecting planets with an orbital plane near our line of sight to the star.

References
Vagnozzi, S. 2016, 51st Recontres de Moriond, Cosmology, At La Thuile