Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 10/04/2013 01:30:01

Title: How do scientists calculate how far back in time events occurred?
Post by: thedoc on 10/04/2013 01:30:01
Tom Brown  asked the Naked Scientists:
As the objects in the universe that we can observe only give us information about the past, and the distance in the past depends upon the distance that objects are from us.

How do scientists compensate for this when they generate equations to define the universe as it is at a given moment ?


What do you think?
Title: Re: How do scientists calculate how far back in time events occurred?
Post by: yor_on on 14/04/2013 15:36:53
Ouch, that's a long history and goes back a while.

At the beginning you used earths relative motion to see how stars, apparently, 'moved' with earths position relative the sun, called parallax, to define a distance. To define those you first have to know the distance from Earth to the Sun, called one astronomical unit (one AU). then you use trigonometry to find a distance to the closest stars. As soon as we could send up probes in space we started to use them too to get those distances better defined, as you won't have a atmosphere distorting in space.

"Some of the best data on stellar positions in the sky come from Hipparcos, a spacecraft launched in 1989 by the European Space Agency. Hipparcos has measured the trigonometric parallaxes of about 10,000 stars to an accuracy of better than 10 percent, out to a distance of about 300 light-years. But our galaxy is about 100,000 light-years across, so parallax measurements become useless long before we approach the distances to other galaxies." from How do astronomers measure the distances... (

To get further out, we make some assumptions, as you can read in that article too. The assumptions are about Cepheid stars being stable objects, presenting themselves astronomically the same way no matter the distance.

"Early in this century Henrietta Swan Leavitt discovered that the longer the period of variation of a Cepheid variable, the greater its luminosity. Another American astronomer, Harlow Shapley, then was able to correlate the brightnesses of Cepheids with those of known types of ordinary stars, tying Leavitt's relative distance scale to an absolute one. "

She should have had a Nobel Prize for that as I feel, but? Anyway, let's hope the Nobel committee are more open minded, and fair, those days.

But you can use all stars if you know their 'spectral class', meaning what their constituents are.

"For stars so far away that their parallaxes are not measurable (yet), mainly photometry is used, that is, measurements of their brightnesses. We know from physics that brightness decreases proportional to the square of the distance (double the distance of an object, and its brightness will go to one quarter); this connection between brightness and distance is known as the inverse-square-law. It has to be changed slightly if one takes General Relativity and the expansion of the universe into account, but these changes are irrelevant for all stars in our galaxy and in other nearby galaxies.

Using this law, if we know the brightness of a star in a fixed reference distance, and compare this to the measured brightness, we can say how far away the star is. The standard reference distance used in astronomy is 10 parsec; the brightness a star would have in this distance from us is called the absolute brightness of the star. Its actual brightness, which we can observe on earth, is called the apparent brightness.

If one examines the stars in the classes further, one sees that each spectral class is associated with a certain color of the stars, and we know from the theory of radiation that the color depends on the surface temperature of the star (this is not only true for stars, but in everyday life, too: if one heats up iron, for example, it first glows red, then yellow, and heating it up even further, it glows white). The temperature is highest for stars in the spectral class O and decreases until the class M; our own sun, which is in class G, has a surface temperature of 5,780 Kelvin (this corresponds to approximately 5510 degrees Celsius or 9950 degrees Fahrenheit). The color is white in class O and goes over blue and green to yellow (class G, our sun) and then on to orange and finally red (class M).

After all of these introductory words, now comes the interesting part: if one plots the stars for which we know the distance from their parallaxes, and hence for which we know the absolute brightness in a diagram, where one axis is spectral class/color/temperature, and the other is absolute brightness, one gets (mainly) a fairly narrow line! Hence spectral class/color/temperature and absolute brightness are strongly related to each other, and if one knows the spectral class/color/temperature, one can easily determine the absolute brightness." From Determining Distances to Astronomical Objects by Björn Feuerbacher. (

If you read those two, especially the last, you will know more about this than 96 % of humanity, and it should take you approximately no more than half an hour :)

Knowledge is the lever that moves our Earth.

and my spelling sux.
Title: Re: How do scientists calculate how far back in time events occurred?
Post by: syhprum on 14/04/2013 21:18:56
May I add that another way of determining the distance of more remote objects is by finding at what speed they are receding from us by measuring the shift in spectral emission lines.
There is a well understood correlation between recessional velocity and distance .
72 km/s per mega parsec (Hubble's law).
Title: Re: How do scientists calculate how far back in time events occurred?
Post by: yor_on on 15/04/2013 18:59:28