h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

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Ever wonder how they figure out how far a star is away from us.



The parallax of a star is one-half the angle at the star. Thus the parallax is the angle at the star in an Earth-Sun-star triangle. Since this angle is always very small, the sine and tangent of the parallax are very well approximated by the parallax angle measured in radians. Therefore the distance to a star is

D[in cm] = [Earth-Sun distance in cm]/[parallax in radians]

Astronomers usually say the Earth-Sun distance is 1 astronomical unit, where 1 au = 1.5E13 cm, and measure small angles in arc-seconds. [Note that 1.5E13 is computerese for 15,000,000,000,000] One radian has 648000/pi arc-seconds. If we use these units, the unit of distance is [648000/pi] au = 3.085678E18 cm = 1 parsec. A star with a parallax of 1 arc-second has a distance of 1 parsec. No known stars have parallaxes this big. Proxima Centauri has a parallax of 0.76". [The double quote is used to denote arc-seconds (as well as inches).]

The first stellar parallax (of the star 61 Cygni) was measured by Friedrich Wilhelm Bessel (1784-1846) in 1838. Bessel is also known for the Bessel functions in mathematical physics.


So that's how its done.

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*walks through reading the last posting from Wayne and bumps into the finishing post*


Oh.. I Win

Well it was what I was reading to recover from that screaming bit awhile back...


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See it calmed you down


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Here go read the rest of it.......

http://www.astro.ucla.edu/~wright/distance.htm



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I win.

*puts cold compress on the lump on Lil's forehead, which was caused by walking into the winning post*

Thank you Ivan such a gentleman as always


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I liked this bit as well......


Tully-Fisher Relation
The rotational velocity of a spiral galaxy is an indicator of its luminosity. The relation is approximately
L = Const * V(rot)4

Since the rotational velocity of a spiral galaxy can be measured using an optical spectrograph or radio telescopes, the luminosity can be determined. Combined with the measured flux, this luminosity gives the distance. The diagram below shows two galaxies: a giant spiral and a dwarf spiral, but the small galaxy is closer to the Earth so they both cover the same angle on the sky and have the same apparent brightness.
But the distant galaxy has a greater rotational velocity, so the difference between the redshifted and blueshifted sides of this distant giant galaxy will be larger. Thus the relative distances of the two galaxies can be determined.

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Oodle flip Wayne

The only parallax I know is a or a


I Win ..... again!

But wait, there's MORE! Pulsar dispersion measures and interstellar extinction increase with distance along a given line of sight and can be used to determine distances. The peak luminosity of a classical nova can be estimated from its rate of decay, but the variation has the opposite sense to that of Type Ia SNe: more luminous novae decay more rapidly. The globular cluster luminosity function can be used to estimate the distance to a galaxy from the observed brightness of its globular clusters.

and thats just part of the first page of 4

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Yes dear


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I win.

Radioactive Dating of an Old Star

A very interesting paper by Cowan et al. (1997, ApJ, 480, 246) discusses the thorium abundance in an old halo star. Normally it is not possible to measure the abundance of radioactive isotopes in other stars because the lines are too weak. But in CS 22892-052 the thorium lines can be seen because the iron lines are very weak. The Th/Eu (Europium) ratio in this star is 0.219 compared to 0.369 in the Solar System now. Thorium decays with a half-life of 14.05 Gyr, so the Solar System formed with Th/Eu = 24.6/14.05*0.369 = 0.463. If CS 22892-052 formed with the same Th/Eu ratio it is then 15.2 3.5 Gyr old. It is actually probably slightly older because some of the thorium that would have gone into the Solar System decayed before the Sun formed, and this correction depends on the nucleosynthesis history of the Milky Way. Nonetheless, this is still an interesting measure of the age of the oldest stars that is independent of the main-sequence lifetime method.





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Contrary to popular opinion, Einstein did not invent relativity. Galileo preceded him. Aristotle had proposed that moving objects (on the Earth) had a natural tendency to slow down and stop. Galileo objected to Aristotle's hypothesis, and asked what happened to an object moving on a moving ship. Now it is still moving in its final state. Galileo proposed that it is only relative velocities that matter. Thus a space-time diagram can be transformed by painting it on the side of a deck of cards, and then skewing the deck to one side -- but keeping the edges along a straight line. Straight worldlines (unaccelerated particles) remain straight in this process. Thus Newton's First Law is preserved, and non-accelerated worldlines are special. This Galilean transformation does not affect the time. Thus two observers moving with respect to each other can still agree on the time, and thus the distance between two objects, which is the difference in their positions measured at equal times, can be defined. This allowed Newton to describe an inverse square law for gravity.

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The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point. The distinction between the whole Universe and the part of it that we can see is important.

In other words the Universe was and always will be there it's all the stuff that's in it that came from the Big Bang.


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I've driven everyone else off


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