Other Stars Like the Sun

Announcements: Astronomy in the news! Even in the Daily Iowan! A report of possible sedimentary deposits on Mars. Check sky and telescope news notes at

http://www.skypub.com

See also dissenting opinions about olivine minerals in Syrtis Major. A second news note was the discovery of a large apparent Kuiper belt object.

Final note is to watch the Moon move between Jupiter and Saturn this weekend.

Stars like the Sun

Last time we discussed what attributes of other stars would indicate that it is like the Sun. We listed temperature, mass, luminosity (how many Watts it radiates), and age.

Astronomers have searched the nearby stars for such similar objects, 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 talking about stars, we need to define a new unit of distance. Since stars are extremely distance compared with the planets, we need a bigger unit than the astronomical unit. The units which are used are the parsec, which is 206265 astronomical units, or the light year. There are 3.26 light years in a parsec.

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.

>>>>>>>>> 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.

We can take this list of stars, and plot them up on a graph in which temperature is one axis, and luminosity is the other.

>>>>>> HR diagram for 99 solar analogs.

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. 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!)

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.

The History of the Solar System

At this point, we can pull a lot of material we have learned about the solar system together to give an overall view of its history. A lot of the facts below have been mentioned before. Now we are pulling things together.

1. How long ago? This was talked about all semester. From rock formation ages we know that the earliest rocks formed 4.50 to 4.60 billions years ago. We have never found a rock older than 4.6billion years.

2. What happened? Look at Figure 13.9, which gives the history in cartoon form. The idea that the Sun formed from a giant, contracting gas cloud is not only based on fundamental physics; we can see this going on out in space at the present time. The winter sky contains two such spectacular regions, the Orion Nebular and the Taurus-Auriga star formation region.

3. The initial spin or rotation of this cloud would have caused it to flatten into a disk, with the largest concentration of matter being at the center. >>>>>> blackboard drawing of disk configuration.

4. At the time the disk formed, it would have been entirely gaseous. How do we have the present solid objects (at least in the case of the terrestrial planets)? Look at the night sky in the next few nights, identify the planets, and think about how the solar system (outside the Sun) is characterized by relatively few, isolated objects.

5. Planetesimals In the gaseous disk of the early solar system, small particular material would have formed out of the gas, in a way very similar to the formation of rain and snow particles from water vapor. These particles would have stuck to other particles to form bigger objects. After enough time, these would have formed rock-sized objects. >>>>>>> copy of painting by Hartmann of the “good ole days”.

6. When these things formed rocks, gravity got into the act, and these planetesimals formed into bigger planetesimals. Eventually the pieces got concentrated into a few places which we now identify with the major planets. The gravitational force of the major planets “vacuum cleaned” the solar system of the planetesimals. The last sweep-up formed the age of bomabardment.

7. A few of the planetesimals hung on however, and are still out there. Many of the asteroids are probably planetesimals that never got gathered up into a major planet. This is also thought to be the case with the small moons of Mars, the satellites of the outer planets, and (most primitive of all) the Kuiper Belt objects.