Other Stars Like the Sun

At this point in the course we have completed the discussion of our solar system, the objects that it contains, as well as the fundamentals of its origin.

A very natural question is: are there other solar systems out there, possibly with inhabitable terrestrial planets.

At this point, we don’t know. Even with advanced astronomical instrumentation, we can’t see other star systems in enough.

The problem: distance. The distances to other star systems are measured in parsecs or light years.

1 parsec = 206265 astronomical units.

1 light year = distance light travels in a year. 1 parsec = 3.26 light years.

The nearest star is 1.3 parsecs away.

Even with modern astronomical telescopes, we can’t see an Earth-like planet around the nearest star. However, this is virtually certain to in the next few decades.

While we are waiting for the technology to detect other Earths, we can be busy in drawing up a list of stars that are like the Sun. If we are looking for other solar systems like ours, it makes sense to star with finding systems where the most important object is like ours.

Question to ask yourself: We are looking for other stars like the Sun. What attributes do we choose for comparison?

The answer to this question lies in the examination of a plot called the Hertzsprung-Russell Diagram. It is a plot of the temperature of stars versus their luminosity, or intrinsic brightness, as shown below. One might expect this to be a “scatter plot”, or look like someone fired a shotgun at a piece of graph paper.

Despite a naïve expectation that stars might be evenly distributed across such a plot, they aren’t. The real situation is shown below. The abscissa (x-axis) is essentially a measure of temperature, and the ordinate (y-axis) is a measure of the luminosity.

The thin band across this diagram is referred to as the Main Sequence of stars. The Sun is a main sequence star. If we want to find stars similar to the Sun, we need to find stars with very similar temperature, and very similar luminosity.

Astronomers have searched the nearby stars for objects similar to the Sun, and they have found many of them. The search has been greatly aided by a spacecraft termed Hipparchos which permitted very precise distances to stars.

In defining stars like the Sun, astronomers have defined three classes of stars in increasing degree of similarity to the Sun. These are

Solar-like stars. This is a general category and would include stars approximately like the Sun. In technical terms, this would be class F and G main sequence stars.
Solar analogs. These are much more similar to the Sun as regards surface temperature, luminosity and age.
Solar Twins. This would be a star which would be identical (as far as we can tell) to the Sun in all of its characteristics. If omnipotent aliens swapped it for the Sun one night, you wouldn’t be able to tell the difference the next morning.

So far astronomers have compiled a list of 109 such stars.

One thing to emphasize is that these are only the nearest such stars. Near stars will be brighter, and we will be able to tell with greater certainty that they are like the Sun.

You can see a famous solar-type star in the night sky these evenings. Get out your SC1 chart and go to a place where the sky is dark. You don’t need a telescope for it; it is a naked eye object. It has a Right Ascension of about 1h45m, and a declination of –17 degrees. The Greek letter Tau looks pretty similar to a “T”. Tau Ceti is a G8 star, whereas the Sun is a G2.

To cull down the list and get really precise, to find the “Solar Twins”, you can carry out more detailed measurements and analyses. We can measure and examine in detail the spectra of stars and compare them with the Sun. The spectrum of the Sun is shown below.

We can compare other stars with this spectrum as regards to its shape, the wavelength at which is it brightest, and the strengths of the absorption lines.

Astronomers know how to analyse these spectra of determine the relative abundances of heavier elements such as carbon, oxygen, nitrogen, iron, etc. We find ranges in such abundances. When finding solar twins, we want stars that are as close as possible as the dear old Sun.

Of these stars, there are a couple which seem to pass muster as a “Solar Twin” .

The one which is closest is 18 Scorpii, or HR6060.

Right now, is not visible, because the Sun is in that part of the sky. In a couple of months, it will be visible in the eastern sky just before dawn. It is just a little too faint to be seen with the unaided eye, at least from in town. It is easy to find in binoculars if you know what you are looking for. A finder chart is given below. Use your SC1 chart to find the constellations of Scorpius and Ophiuchus, then use this chart to narrow your search.

http://www.astronexus.com/scripts/eos/eos_star.php?IDType=HD&ID=146233

A picture of this star is shown below. Given what we have learned this semester, one can speculate on whether this star is also surrounded by planets, asteroids, comets, etc, and in fact life, like our solar system.

Like all great things, 18 Scorpii has its own web page. This is given at

http://www.solstation.com/stars2/18sco.htm

In the near future, say 10 – 20 years, we will get precision data on many more stars, and will find many other solar twins like 18 Scorpii. This is essentially a certainty, as more powerful telescopes in space allow us to find thousands, and eventually millions of solar analog stars. More will be said about this in the concluding two lectures of the course. These stars will get intense scrutiny for the possibility that they may harbor solar systems like ours, including terrestrial planets like the Earth.

Planets around other stars: the Hot Jupiters

The above description of stars like the Sun naturally leads to the question of whether we can detect planets around them now. Ten years ago that was considered to be impossible, but a surprise discovery was made in 1995. It emphasizes the point that nature is often much more surprising than we can imagine. The following provides a link to a lecture I recently gave about exoplanets.

http://phobos.physics.uiowa.edu/~srs/RareEarth/Exoplanets_files/frame.htm