Distances to the Stars 

1. How far are things?
   ....and How do we find out? They don't make tape measures that long, right?
2. Parallax
3. The Light-Year
4. Spectra

Just how far are planets, stars, galaxies, and other things in space?

• Start here: An Atlas of the Universe.
• AND Check out Powers of Ten: Wait for the Java Applet to load, click Manual so you can control your speed, then note the distances on the lower right as you click through the  pictures.

Relative Distance Factoids:

• The Earth is an average of 93 Million miles from the sun. It takes LIGHT about 8 minutes to get to us from the sun. If you were able to simply direct the Space Shuttle out of orbit at its average orbital speed of 25,000 MPH straight toward the sun, it would take you about 5 months to get there (assuming we are not moving and could just fly that distance. The reality is a little more complicated).
   
• The nearest star, Proxima Centauri, is about 4.2 light-years away. If you were able to simply direct the Space Shuttle out of orbit (as above) at its average orbital speed of 25,000 MPH straight toward Proxima Centauri, it would only take you about 109,000 years to get there. "Thet's a fer piece, Butch!"

You should be able to see that the distances outside of our solar system are ASTRONOMICAL!

I don't know about you, but I cannot fathom those distances. I can't even grasp the 93 Million miles to the sun!

 So don't feel that you can't deal with these large numbers. Sometimes you just have to accept something and go with it. Greater understanding will usually come later.

So, let's summarize:

• The Sun is 8 light-minutes away (on average, we almost always have to use an average number)
  • The sun is part of our Solar System.
• The next nearest star is 4.2 light-years away.
  • This star, Proxima Centauri, is outside of our solar system.

  Remember that our solar system, and other stars like Proxima Centauri, are in a galaxy we call The Milky Way?

  HOW FAR AWAY are other Galaxies that are outside of the Milky Way?
   

• The next nearest large (or major) galaxy, the famous (and beautiful) Great Galaxy in Andromeda, is about 2.2 Million light-years away (according to NOAO).
  • Adapting the scenarios above, how long do you think it would take you to get there? (Try The Light-Year down this page to see the numbers, then do the math, and get back to me)

• Shift in position of an object when viewed from 2 angles

• If you know the baseline distance, and you can measure the angle of the shift between the two images, then you can figure the distance to the object.

What is the limit to parallax?

• Used only for "nearby" stars - those within a hundred parsecs from Earth's surface, (A parsec (abbreviated pc) is about 3.26 light-years), so 300 light-years or so is the maximum.
• Hipparcos (a satellite) has made it possible to determine parallax out to 1,000 pc or more.
  • As technology improves, this limit may lengthen.

Links
for parallax explanations:

• The Hipparcos Space Astrometry Mission
• Determining distances through parallax
• Parallax Movie
• Aristotle and parallax - click on "parallax" after reading the paragraph.
• NASA parallax explanation
• Definition at infoplease

Light travels at a constant speed!
(This has been experimentally proven.) 

Chandra

 Get It?

(Sorry for the redundancy, but lots of people just don't GET IT!)

FOR EXAMPLE:

1 Light-Year

 =      ~300,000 km/sec  (the speed of light)
 [~186,000 miles/sec]

x       ~31,536,000 sec/yr (seconds in a year)

= ~9,460,800,000,000 km  (9.5 x 10¹²)

(9.5 trillion km)

Conversion, miles::kilometers

Conversion LINK:  OnlineConversion.com

1 Km = 0.6 Miles  (Rounded)

1 Mile = 1.6 Km  (Rounded)

In Science fiction you often hear them use the parsec as a measurement.

It is a previously more common measurement of astronomical distances,
equal to 3.26 light years, still used by professional astronomers.

Its name derives from the parallax-second.

You will find it used a lot in the professional literature, though not so much in the popular.

The scientific symbol for the speed of light is C (the capital letter C).

The closest star other than our sun (Proxima Centauri) is 4.2 light-years away,

~ 40 x 10¹² kilometers !   (It’s easier to write down, and think about, 4.2ly than it is 42,000,000,000,000 km, don't you think?)

If you were able to travel the speed of light, it would take you 4.2 years to get there! 
So if you had a ship much faster than what we are able to build now that could go as fast as, say, 0.1 C, one-tenth the speed of light, it would take you 42 years to get there!   Better pack an extra lunch!

Stellar Spectra

Many stars are way too far away to use parallax to find their distances.  (See question above).  So, we must use a method called finding "spectroscopic distances."  From these spectra we can determine the class of a star and so its luminosity. Comparing this with the apparent magnitude tells us the distance, after a little math.

• The component colors of light we might otherwise see as white,
     or some other single color, are seen as a spectrum.

• Part of the electromagnetic spectrum *

• Looks like the rainbow colors transmitted by a prism.
   
• Absorption lines (dark lines of no color) appear, indicating presence of certain elements in star's atmosphere that absorb light at particular wavelengths.*
   
• Indicators of the star's composition (elements that absorbed light)
   
• So, we can also find distances to these far away stars!*

* The link takes you to the SPECTRA site, Created by Jim Kaler , Prof. of Astronomy, University of Illinois:

The Doppler Shift

Visit these sites for an explanation...

The Doppler Shift from Cornell U.
Doppler Shift due to wobble
Orbiting Binary Stars animation

Return to Constellations

MY answer to How long would it take to get to the Andromeda Galaxy at speeds possible with our current technologies is: about 57 Billion years!!! (Go ahead and check me on this. These are large numbers, I only ran them once)