How do scientists measure the distance of a star?

Question

How do scientists measure how far a star or a galaxy is? and is that fool proof?

For e.g if a big star is far off and a smaller star is near by and they might look similar in intensity and size to naked eyes, but do scientists really have methods to figure out their reals distances.....assume that both of those stars are moving away from us at the same velocities...

Answer 1

The full answer to this question is pretty complicated, because different methods are used for different types of stars in different circumsances. An entire year-long college course could be devoted to the answer. But here is an outline of the process:

1) The distance to objects within the Solar System is primarily determined using NASA probes that actually visit the planets, combined with a very complete computer model of the solar system compiled and put together by NASA. This determines the first step in the distance "ladder", the size and shape of the Earth's orbit.

2) The nearest few thousand stars have distances measured by the method of parallax. As the Earth goes around the Sun, the nearby stars seem to move in a little ellipse compared to the distant stars. (If you hold out your finger at arm's length, and blink one eye then the other, your finger will seem to move against the background.) This measurement can be made very accurately for nearby stars, and since we know the size of the Earth's orbit, we can calculate the distance to those stars accurately.

3) The few thousand stars with parallax distances have a lot of different sizes and brightnesses. By studying those stars in detail, we find that the spectrum of the star is related to its brightness. So now, if we take a spectrum of a more distant star, we can tell how bright it actually is (absolute magnitude), and by comparing that to how bright it looks, we can tell how far away it is. This is called the method of "spectroscopic parallax". It is accurate to maybe a factor of two, for any one star.

4) Some stars are in clusters. We assume that all the stars in a cluster are about the same distance away. If we can get spectroscopic parallaxes for some of them, then we can get the brightness for all of them. If we know the brightness of a type of star, and we can recognize that type by some other means (spectroscopy or photometry), then that type of star becomes a "standard candle" and the apparent brightness of a standard candle star can be used to determine its distance.

5) Using the previous methods, we find that certain rare types of star get brighter and fainter over days or weeks in a regular way, and that the period of those variations is directly related to their brightness. This is the "method of variable stars", and because these stars are quite bright, it can be applied to stars that are very far away---basically anywhere in the Milky Way, and in other nearby galaxies as well. So now we know how far away the nearby galaxies are.

6) In the nearby galaxies, sometimes supernovae happen. A type of supernova, type I, seems to be the same brightness in all galaxies. These supernova can be seen in much more distant galaxies as well, so now we know the distance to more distant galaxies.

7) We can measure the velocity of galaxies using the Doppler shift in their spectrum. Using all the previous distance methods, we find that the more distant galaxies are moving away from us faster than the nearby galaxies. This is called the "Hubble flow", and the ratio between velocity and distance is the "Hubble Constant". With this rule, we can now get the distance to any object in the Universe whose velocity can be measured from its spectrum. That's how we know the distance to Quasars and very distant galaxies.

Answer 2

There are several different methods, depending on how far away the star or galaxy is. The farther away it is, the less certain the measurements, because the precision of the techniques for the farther objects depends on the precision of the techniques for the closer objects.

We can measure the distances to planets using radar ranging - bounce a radar beam off a planet, measure the time it takes for the beam to return. Since radar beams travel the speed of light, we can then measure the distance to the planet. Indirectly this also gives us the distance to our nearest star, the Sun.

The nearest stars (aside from the Sun) are measured using stellar parallax - as we orbit the Sun the nearby stars appear to shift back and forth relative to farther stars. We can measure this shift, and that plus knowing the distance to our Sun can give us the distance to the star. This works out to around 300 light-years, I think. Still well within our Galaxy.

The techniques for farther stars get a bit complicated, so I'll just list some of them and you can look them up if you are interested - also search for "distance ladder"

- Stellar parallax (as mentioned above)
- Spectroscopic parallax (which has nothing to do with stellar parallax)
- Variable stars (Cepheid variables, RR Lyrae stars)
- Novae and supernovae, and other standard candles
- Hubble's Law

There's more, but that's all I can think of off the top of my head.

Answer 3

Just to add to what Kris said, a number of the distance measuring techniques involve figuring out what the star's actual (absolute) brightness is. Since we know that the intensity of light decreases proportional to the square of the distance, comparing the absolute brightness to the observed brightness enables us to calculate the distance to the star.

This is the technique used with Cepheid variables and other "standard candles". It also works with large star clusters, as the distribution of spectroscopic types makes it possible to determine absolute magnitudes. It is often not possible to estimate the distance of a solitary non-variable star further than 1500 light years.

Answer 4

The distance to a star is measure by using the red shift of the light spectrum. In physics and astronomy, redshift is a phenomenon in which the visible light from an object is shifted towards the red end of the spectrum. It is an observed increase in the wavelength, which corresponds to a decrease in the frequency of electromagnetic radiation, received by a detector compared to that emitted by the source. The corresponding shift to shorter wavelengths is called blueshift.

The phenomenon goes by the same name even if it occurs at non-optical wavelengths (e.g. gamma rays, x-rays and ultraviolet). At wavelengths longer than red (e.g. infrared, microwaves, and radio waves) redshifts shift the radiation away from the red.

Redshift typically occurs when a light source moves away from an observer, analogous to the Doppler shift which changes the frequency of sound waves. While observing this redshift has a number of terrestrial uses (e.g. Doppler radar and Radar guns), it is famously employed in astronomy where it is used as a diagnostic in spectroscopic astrophysics to determine information about the dynamics and kinematics (i.e. movement) of distant objects. This redshift phenomenon was first predicted and observed in the nineteenth century as scientists began to consider the dynamical implications of the wave-nature of light. There is also a gravitational redshift which happens due to the time dilation that occurs in general relativity near massive objects. Most famously, redshifts are observed in the spectra from distant galaxies, quasars, and intergalactic gas clouds to increase proportionally with the distance to the object. This is generally considered to be one of the major forms of evidence that the universe is expanding, as predicted by the Big Bang model.

Answer 5

To Kris' list add the Distance Modulus.

Answer 6

this is a universal fact that earth is milloins of kilometer's far a part from the earh surface .

as it is difficuilt for oneself to calculate such a long distance so scientists discovered a unit called light year where



one light year = 9.46 into 10to the power of 12 km

{or}
9.46 into 10to the power of 15 mts

Answer 7

by calcualating parallactic shift of the background from two or more positions of earth on its orbit.

Answer 8

It s brighness, size etc help to determine. ALso by focusing telescopes one could estimate the distance from the focal point etc

Answer 9

with a very long tape measure?