29:50 Modern Astronomy
Fall 1999
Lecture 18 ...October 6, 1999
Stars Like The Sun

(1) Tonight, if clear, meet out in the lobby and we'll go to the roof and look at stars.
(2) Astronomy in the news: a couple of days ago the Crab Nebula was in the New York Times
Put image of Crab at X-rays here

Stars like the Sun
A naturally curious person, looking at the night sky, will wonder which of those stars might be like the Sun. There are a number of reasons for wondering this, but the most obvious is to wonder if those stars are also homes to habitable planets and life forms.

Question for audience: How would we go about determining which stars are similar to the Sun? One kind of quantities would we compare?

The right answers, given what has been discussed in class this semester, would be that we would look for a star with similar spectral type (or equivalently temperature) and luminosity (or equivalently absolute magnitude) to the Sun.

With this list, you could go on to measure more quantities, and try a more stringent comparison. The observational technique you would use would be high resolution spectroscopy, in which we really spread out the light and examine in details the shapes and strengths of the absorption and emission lines.

People have done this and compiled a list of about 109 (fairly nearby) stars which seem to be quite similar to the Sun. The goal of this program, in the words of one of the scientific leaders is ``To determine whether photometric solar analogs (in other words, stars which seem similar on the basis of their color and absolute magnitude) remain such when submitted to detailed spectroscopic analysis''.

With high resolution spectroscopy, one can determine a whole host of stellar properties than can be compared, such as atmospheric structure, mass, chemical composition, age, luminosity, magnetic field, and rotation .

Scientists studying these stars have made three interestings classification categories.

1. Solar-Like Stars. This is a general category, and would include stars approximately like the Sun. That is, spectral class F and G Main sequence stars.
2. Solar Analogs. These would be much more similar to the Sun as regards surface temperature, luminosity, and age.
3. ``Solar Twin''. This would be an object which would be identical to the Sun in all of its characteristics. If omnipotent space aliens swapped it for the Sun one night, you wouldn't and couldn't tell the difference when you woke up.

Let's look at the candidates and talk about how astronomers grouped 109 candidates into the various classes, and what they found out about these stars.
Part of Table with solar-like stars.

There are a couple of interesting things to note about this table. (a) The spectral types are all very similar to the Sun. (b) The apparent visual magnitudes are usually about 5 - 6. This should tell you something about their distances.

Transparency with CaII line spectra.
This transparency shows high resolution spectra of Calcium absorption lines of the Sun (top) and four stars from this sample. They are all clearly very similar, but amazingly one can measure slight differences between the stars.

As I have mentioned, the Hertzsprung-Russell diagram is an important tool for astronomers. The next transparency
HR diagram for solar-like stars
shows that these stars (most of them) are indeed very similar to the Sun. There is an inner bunch that are clearly very similar.
Blow up of HR diagram around the Sun.
One can have a pretty good idea from this which stars are older than the Sun and which are younger.

You can go on refining this sort of analysis, are try and get better measurements of fundamental stellar parameters. This has been enormously helped by the Hipparchos spacecraft, which vastly improved the distances (think why greatly improved distances to the stars would make an analysis of this sort more precise). This has enabled astronomers to look for real Solar Twins.
Abstract of ApJ paper, read it.

The star 18 Scorpii at a distance of 46 light years (14.1 parsecs) is the most similar star known to the Sun. The position of this star on the HR diagram is very similar to the Sun
Transparency of HR diagram with , 18 Scorpii, HR2290, and 16 Cygni.

An oddity is that a number of the Solar Like stars, and Solar Analogs are known to have planets. Examples are 16 Cygni, 51 Pegasi, 70 Virginis, 47 Ursa Majoris, and Iota Horologii. The problem is that the planets that have been found are big, goofy planets. The most common types are the ``hot Jupiters''. Planets with the mass of Jupiter (or more), but very close to the Sun. Obviously the Sun has no objects of this sort. When these planets began to be discovered 4 years ago, they not only were unexpected, they were contrary to expectations. See p 632 of your textbook for further description of extra solar-system planetary systems. The number is increasing all of the time.

• About this document ...