Other Stars Like the Sun

News notes on astronomy. Check Sky and Telescope web page, with news on Killer Asteroids http://www.skypub.com

● If you are going to be in a dark-sky setting in the next couple of weeks, and have a pair of binoculars, trying seeing the asteroid Vesta. This is your golden opportunity to see an asteroid. The finding chart is at:

http://www.skypub.com/sights/images2001/0111vesta_big.jpg

● Maximum of Geminid meteors will be next Thursday night. They may be seen to start several days ahead of maximum, so keep your eyes open this weekend.

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 detail.

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 change before you turn into old coggers.

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 for the august assembly: We are looking for other stars like the Sun. What attributes do we choose for comparison? Personality?

Brief foray into stellar astronomy….The Hertzsprung Russell diagram.

>>>>>>> Blackboard drawing of HR diagram, indication of where the Sun is.

It is useful to introduce a shorthand notation for stellar classes, such as O,B,A,F,G,K,M.

The Sun is classified as G2V.

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.

Question for the august assembly: why would a very precise distance allow us to determine if a star was like the Sun?

In defining stars like the Sun, astronomers have defined three classes of stars (Hey! This is science; you can’t talk without forming classes!) 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 terms of 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 Twin. This would be a star which would be identical (as far as we can tell) to the Sun in all of its characteristics. If ominipotent 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.

>>>>>>>>> HR diagram with solar-type stars

>>>>>>>>> Table of solar-type 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 one of these solar-type stars right overhead these evenings. It is Mu Cassiopiae, and is a solar-type star 7.34 parsecs away. A bit to the west you can find the star 16 Cygni, which is a double with two solar type stars.

You can see they adhere pretty closely to the Sun. A magnified version of this has the names of stars which have been shown to have planets (more below). UFO nuts say that Zeta Reticuli was the home star system of the nasty aliens who abducted the couple in “interrupted journey”. (Be sure and write this down. It might be on the final exam!)

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.

>>>>>> Spectrum of the Sun.

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.

>>>>> HR diagram of solar analogs.

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.

>>>>>> Abstract from the Astrophysical Journal.

Planets around other stars: the Hot Jupiters

Nature usually turns out to be stranger and more interesting than our expectations.

http://www.obspm.fr/encycl/catalog.html

http://cannon.sfsu.edu/~gmarcy/planetsearch/multi_panel.jpg