29:51 Introductory Astronomy Lab

Spring 2001

Experiment 5

Week of April 16, 2001

Stars Like The Sun

One of the most intriguing thoughts a person can have while looking at the starry sky is that those objects are like our Sun, and possibly have planets around them. While all stars resemble the Sun, more or less, stellar astronomy has progressed to the point that it can identify those stars most like the Sun as regards temperature, luminosity (intrinsic brightness), age, and mass. This was discussed in the class last semester in the lecture on December 6. The notes are still on line; check them out!

As discussed in that lecture, stars like the Sun are grouped into three classes in order of increasing similarity to the Sun. These classes are solar-type stars, solar analogs, and solar twins. In the class this week, we will find and observe three examples of solar analogs. The scientific classification term for the Sun is a G2V star. A larger number means the star is somewhat cooler than the Sun.

Stars like these are of great interest to astronomy and space science right now. The US space agency is planning a mission called the Terrestrial Planet Finder, and the European space agency is planning a similar mission called Darwin, which may be launched in about ten years. The ambitious goal of these missions is to search solar analog stars for planets like the Earth, and determine if there is life on those planets! The stars you will be observing this week will be high on the list to be examined.

There will be two components of this observing project. The first will be to find these objects, using binoculars and star charts. As seen through binoculars, they will seem like fairly ordinary stars. However, by observing them you will appreciate something which frequently occurs in astronomy and science in general; an object which at first glance appears somewhat ordinary becomes of immense interest when you realize something of its background and nature.

In the second component of the project, we will use the Celestron telescope to amplify the light of the stars, and see if we can see the lovely yellow glow of life-bearing solar stars. The list of our stars is given below. The columns give the catalog number of the star (the so-called HD number), the spectral type, the apparent visual magnitude, the right ascension, the declination, and the distance in light years.

Star	Spec. type	V Mag	RA	DEC	Distance
78366	F9V	5.93	09h09m	33d53m	62 ly
86728	G2V	5.35	10h01m	31d55m	48 ly
84737	G1V	5.09	9h49m	46d01m	60 ly
76151	G3V	6.01	8h54m	-05d26m	56 ly

Binocular Observations

Begin by plotting the positions of these stars up on the SC1 chart to see what part of the sky we are examining. Note on the chart the “sickle” of constellation of Leo. Notice the stars α and 38 Lyncis (above and to the right of the sickle of Leo) and all three stars in the nondescript constellation Leo Minor. Notice also the three pairs of visual doubles in Ursa Major, κ,ι μ,λ , and ν,ξ. All of these constitute important “asterisms” for locating our stars.

Before the night of the main project, it is highly desirable to locate these stars in the night sky. They are in excellent location for observing now in April. At about 9PM the sickle of Leo is due south and at an altitude angle of about 60 degrees.

During daytime you should consult a detailed star chart which shows the project stars. My favorite is the Skalnate Pleso atlas. The idea here is to find the solar analog stars on this atlas, note convenient asterisms which will help find the solar analogs, and make the connection between the sky as plotted on the SC1 chart and that of the Skalnate Pleso. A copy of the Skalnate Pleso will be available when we do our observations.

On the night of the observations find the right part of the sky, find nearby benchmark stars and asterisms, and locate the real stars in the night sky. Think about the properties of these stars, such as their spectral type, affinity to the Sun, and distance, as you are looking at them. After the fact, make notes in your notebook.

Telescopic Observations of Solar Analog Stars

The second part of the project is more ambitious, and seeks to use the Celestron telescopes to illustrate an important physical property of these stars. The reason for making telescopic observations is to utilize the light-collecting capability of the telescopes. The human eye senses color only if the light is bright enough. The goal in this part of the lab is to see the yellow color characteristic of a star like the Sun. To test for color detection, we need some “standards”, stars which are very whitish or very reddish. The table below gives a set of stars in the same part of the sky which are more or less of the same brightness, and should be either white (spectral classes A and B) or orange (spectral class K) or red (class M).

Star	Spec. Class	Mag	RA	Dec
Kappa Uma	A0III	3.60	9h04m	47d09m
Delta Hya	A1IV	4.16	8h37m	5d41m
30 Lyncis	B8	5.32	9h13m	43d12m
38 Lyncis	A2IV	3.82	9h19m	36d48m
HR3834	K3III	4.68	9h38m	4d38m
Pi Leonis	M2III	4.70	10h00m	8d02m

Set up the Celestron and attempt to align it with the pole star. Then use an easy-to-find star (say Gamma Leonis or Alpha Lyncis) to set (calibrate) the setting circles. Check the accuracy of the setting circles by first finding other bright stars in this part of the sky, such as μ or λ Ursae Majoris. Then use the setting circles to find the solar analog stars and the color standards in the above table. As you look at the solar analogs, again think on the nature of those points of light in the telescope field. See if you can perceive the colors of these stars. Moving the telescope back and forth between the color standards and the solar analogs will help. Expect subtle differences of color; not vivid hues like Christmas tree ornaments.

Record your experiences.