In continuation to my question on astronomy?

Question

Many pesons have given answers . What they have said all that I know. Might be It's my fault in not describing the question properly. The question is that the scientists are telling that they have discovered some planet, star or milky way which is 100 billion years away. What I mean to say is that how can tjey tell that so and so star is so much distance away, without actually looking into it (atlest through telescope). To see that far away things, even by telescope it takes so much time (atleast some thousands of years). Hence how can they say?

H.K.Kishore

Answer 1

Cosmo said:

"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."

This is totally incorrect. The distances to other bodies in our solar system were determined by measuring their orbital speed, or rather their apparent motion with earth-based instuments and then applying Kepler's Laws of Planetary Motion to determine the distance (speed is related to distance). In the case of the planets the distances have been known for centuries. Pluto's distance was known shortly after its discovery in 1930.


NASA had absolutely NOTHING to do with determining the disances to planetary bodies. They were already known from previous observations. NASA would not have been able to send a probe to Pluto or the planets had its precise distance not been previously known.

If he will correct his answer, I will delete mine. I gave him the thumbs-up (as I did for others) but not for his first paragraph.

Answer 2

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.


Distant objects can be seen from the light they emitted long ago. You don't have to be watching the whole time the light is travelling. Naturally, if the object is very far away, the way it looks "now" is different from the way we see it. We must always remember that we see distant objects as they were, not as they are.

Answer 3

First, just a little note about the "100 billion (light) years away". I trust that you just put this as an example, the point here is that we have not seen anything further than 14 billion light years away, because 14 billion years is the apparent age of the universe, and 14 billion light years is percieved size.

Now on to your question. Determining the distance to an object in space is based on a few techniques. One of the earliest one was the brightness of cepheid variables, where the brightness and variability period are linked. Thus, observing a cepheid variable of a given period allowed an easy determination of its absolute brightness, and from the apparent brightness we can see through telescope, the distance can be derived. This allowed a fairly good estimation of the size of our own galaxy, and the distance to some close galaxies.

Next is the brightness of type 1a supernova, and the light curve they have over time, again with a relationship between them. Since supernova are rare, this "standard candle" took some time to be developped, but even distance galaxies will occasionally have supernova, and thier distance can then be calculated.

Then came Mr. Hubble, who noticed something called the redshift in galaxies. The further a galaxy, the more shifted to the red (because of the Doppler effect) its light tends to be (there are local variations, for instance, Andromeda galaxy, 2.2 million light years away, is actually on a collision course with ourr own galaxy); but in general, the further a galaxy is the faster it is moving away. So, just by looking at the redshift of a galaxy, a very good estimate of its distance can be obtained.

Of course, what we see now is what was then. If you look at a galaxy that is one billion light years away, you see the light that galaxy put out one billion years ago. Right now (if "right now" means anything in this relativistic universe of ours) it is putting out light that will not reach us for more than one billion years, as it is now possibly further, having moved in the meantime.

So, it was a cascade of measurement. Cepheids to assess the size of our galaxy and the distance to relatively close galaxies which allowed the establishement of the supernovae measurement technique, used for more distance galaxies, wich allowed the calibration of the redshift method, used for most everything else further.

Is this what you wanted to know?

Answer 4

First let us clarify a few misconseptions.

1. To measure distance to stars observation is required and this is done by using telescopes
3. The brightness of the star also cannot really tell us how far away it is because stars vary in size, brightness and distance.
4. Other galaxies are so far away that we cannot distinguish any stars in them so the stars we know and measured their distance are in our galaxy the milky way.

The most popular method of measuring the distances to the stars is the Parallax method. A nearby star's apparent movement against the background of more distant stars as the Earth revolves around the Sun is referred to as parallax.
The parallax can be used to measure the distance to the few stars which are close enough to the Sun to show a measurable parallax. The distance to the star is inversely proportional to the parallax.

Other methods (eg interferometric astrometry)exist but they require specialist knowledge and understanding of the mechanics of space to realise them.

Actually measuring stellar distances is a whole branch in Astronomy called ASTROMETRY. Astrometry dates back to ancient greece and Hipparchus who compiled the first map of stars that he could see about 2000 years ago. From 1989 to 1993, the European Space Agency's Hipparcos satellite performed astrometric measurements resulting in a catalogue of positions accurate to 20-30 milliarcsec for over a million stars.

Answer 5

How do astronomers measure the distances to galaxies?
Astronomers measure the distance to a galaxy in the same way we estimate the distance to an oncoming car by the brightness of its headlights. We know from experience how much light a car's headlights emits, so we can determine how far away the car is.

To measure the distance to a galaxy, we try to find stars in that galaxy whose absolute light output we can measure. We can then determine how far away the galaxy is by observing the brightness of the stars. Such stars can help us measure the distance to galaxies 300 million light years away.

If a galaxy is too far away for us to distinguish individual stars, astronomers can use supernovae in the same manner, because the light output of supernovae at their peak brightness is a known fact. Supernovae can be used to measure the distance to galaxies as far as 10 billion light years away.

Answer 6

The answer is red shift. They use a sort of spectrometer that measure the wavelgnth of the light emited from the stars. Red shift is an indication of how "tired" light gets as it tends to bend over long distances due to gravitational effects. They use one star as a refence, such as our own or another known star, then they measure the test star, they will then compare the data and essential assume some constants and ratio the intensity, versus the wavelngth and shift.

Answer 7

They know because of spectrographic red-shifting. The element hydrogen has a certain unique spectrographic signiture. The distance between us and them is dependent on how far the signiture is from center. If the signiture is is the Blue range, we know it's moving toward us; and if it's in the red range, we know it's moving away from us. The distance is an algorythm calculated off of distances from, say, Andromeda which we do know for certain.

Answer 8

It's simple i mean the theory.U just have to use a spectrometer to find out the wavelength of the light and then it's over.Divide the wavelength by the speed of light.U get the distance.

Answer 9

well I'm not your all time crazy science lover but they have done trials and errors and made guesses to theses things of what you ask I'm sure a bunch of scientists got together and said this is how its going to be and bam there you have it.

Answer 10

They measure the photons.