Midterm Practice Anyone?

4.) The parallax of Betelgeuse is 5 milli-arcseconds = 0.005 ". Determine its distance (including units).

5.) Betelgeuse is the first star whose diameter was directly measured. Its diameter is roughly 10 AU! Using value(s) determined above, calculate the angular size of the star, preferably in arc seconds (that is, the angle made by the left and right edges of the star, as seen from Earth.) Hint: make a sketch and think of a useful triangle.

Parsecs & AU in Binaries

It is useful in measuring the size of binary star orbits!

Recall this triangle from parallax:

Side: 1 Parsec = 206265 AU (d)Side: 1 AU Angle: 1 arc second. (p)

eg.: Binary @ 1pc. Stars are separated by 5”How many AU apart are the stars?

In 1910, it was realized that the star Sirius wobbles periodically...

...an astrometric binary

Position of Sirius over 100+ Years. The primary star (M=2MSun) wobbles a lot due to an unseen secondary.

μ

proper motion

Sirius A & B

So the unseen secondary (“Sirius B”) must have considerable mass.

(Its mass is 1 MSUN, half that of “A”)

Eventually Sirius B was observed.... it is 1000 times fainter than A

It is also much hotter than A.

How can a hotter star be so dim? Sirius B

Sirius A

Siriusα CMa A

Luminosity: 25 LSun

Temp.: 10,000 K

α CMa B

Luminosity: 0.025 LSun

Temp.: 25,000 K

Use Stephan Botzman Law to determine the ratio of Radii.

Sirius BSirius B has a mass of 1 MSUN.

But its radius is about (0.01 RSun ~ REarth) !

Relative to the Sun its density is.....

huge!

It is called a White dwarf

One teaspoon of white dwarf material would weigh 10 tons!!!

This is a new type of matter!

=

Sirius A&B in Xrays: Which is A?