How do they measure a star that is 100 million light years away?? In simple terms please.?

Question

How do they measure a star that is 100 million light years away?? In simple terms please.?

Answer 1

That's about 50 times as far away as the next nearest galaxy to us, M31 in Andromeda.



At that distance, the galaxy which the star is part of would just be a smudge, and a nova or supernova is the only star in it you could pick out as a star. Certain characteristics of its "spectrum" reveal how much matter it contains and what kind of reaction has caused it to go nova. That lets you calculate the absolute light energy which it should be emitting, and the difference between that and what you can actually see is caused only by its distance, which you can then calculate - allowing for dark matter, of course.



After that, you can look at the red shift of its spectrum, caused by the speed it is receding at, and plot another point on Hubble's famous straight-line graph of recession speed versus galactic distance.



With enough of Hubble's graph plotted in this way, you can determine any visible galaxy's distance by the red shift of the light emitted by all its ordinary stars, without waiting for one of them to go nova.

Answer 2

In order to calculate how far away a star is, astronomers use a method called parallax. Because of the Earth's revolution about the sun, near stars seem to shift their position against the farther stars. This is called parallax shift. By observing the distance of the shift and knowing the diameter of the Earth's orbit, astronomers are able to calculate the parallax angle across the sky.

The smaller the parallax shift, the farther away from earth the star is. This method is only accurate for stars within a few hundred light-years of Earth. When the stars are very far away, the parallax shift is too small to measure.



The method of measuring distance to stars beyond 100 light-years is to use Cepheid variable stars. These stars change in brightness over time, which allows astronomers to figure out the true brightness. Comparing the apparent brightness of the star to the true brightness allows the astronomer to calculate the distance to the star. This method was discovered by American astronomer Henrietta Leavitt in 1912 and used in the early part of the century to find distances to many globular clusters.

Answer 3

I gave Dr Anders a star for a great answer, but to sum up



1. For nearer stars in our galaxy, they use parallax – how much does the star seem to jump when they photo it from one side of Earth’s orbit to the other ( 6 months later)



2. Then as mentioned, Cepheid variables – their pulse rate is an indication of their brilliance, and how bright they actually look tells you how far away they are. But of course the first and nearer Cepheids would have to have been measured with parallax (see 1) in order to establish a yard stick



3. Using the Cepheid variables, they can then identify them in the nearer galaxies, and thus work out the distance to the galaxy.



4. Finally, the red shift in the galaxies spectrum is an indication of distance. But again they needed a yardstick, and that was establishing the distance to the nearer galaxies with the Cepheid variable method (3). Then you can relate their red shifts, which will be small, with the red shifts of the more distant galaxies, which will be proportionally larger.



The parallax method (1) is the only one that provides an absolute distance (IE in miles, kilometers or light years). Without the parallax method as a basis, the other methods would only provide a relative distance, eg galaxy M101 is 10 times as far away as M31.

Answer 4

That's a very good question. If our present laws of physics are correct, then light speed is the cosmic speed limit. Ergo, if a star or galaxy is one million light years distant, it follows that the universe must be older than one million years. You are correct. The light from that object would take one million years to reach Earth. Confusion is an atrocity. Alas, I think it's part of the human condition. I walk around confused all the time, and it is disheartening. I don't know the truth in terms of how old the Earth and the universe truly are. However, that modicum notwithstanding, the Holy Bible does contain metaphors. Sometimes, it's difficult for scholars to ascertain which passages are to be interpreted literally and which are to be understood as being metaphorical. I applaud you for this question, because it is a cogent one, and expresses a valid and legitimate concern. But what is the connection between being a Creationist and determining the age of the cosmos (and Earth)? Do you mean to point out the ostensible fact that the Earth is more ancient than some people would believe, and, ergo, evolution is more likely to be fact rather than 'just' theory? Again, I don't know how old Earth is. But to be honest, I do believe that the Earth is much older than 6 or 10 thousand years old. But for me, there is no challenge in believing that the world is older than some Creationists may think. To me, it doesn't conflict with the idea that God created human beings in our present incarnation. But to be honest, it is a good counterargument against the idea of Creationism, provided that one of the characteristics of Creationism is that the Earth has to be only a few thousand years old.

Answer 5

You didn't give enough information to define 'measure'. If you mean measure the distance to a star, then Cepheid variables MAY be used, if any bright enough can be found at that distance. If not, then the brightness of globular clusters can be used (they have average absolute magnitudes).



If it is the type of star, approximate diameter, and so on, they use spectroscopy. With that technique they can determine the type of star (spectral type), how bright it SHOULD be, and then determine (guess at) whatever else they need to find out. If the star is found to be a certain type with a certain luminosity, then the actual magnitude can be used to determine the size of the star (if it's distance is known from other means). Astronomers don't rely on one measurement like this, they use several, and then make educated guesses from all that information.

Answer 6

Trigonometric Parallax



Trigonometric parallax is used to measure the distances of the nearby stars. The stars are so far away that observing a star from opposite sides of the Earth would produce a parallax angle much, much too small to detect. As large a baseline as possible must be used. The largest one that can be easily used is the orbit of the Earth. In this case the baseline is the distance between the Earth and the Sun---an astronomical unit (AU) or 149.6 million kilometers! A picture of a nearby star is taken against the background of stars from opposite sides of the Earth's orbit (six months apart). The parallax angle p is one-half of the total angular shift.

Answer 7

nick-- you can give questions stars but you can only give a thumbs up or thumbs down to answers.



The parallax formulas are used for stars up to 100 million light years away. For stars beyond 100 million light years other systems are used. Your question lies on the border. As parallax has already been explained, I offer complete measurement systems for stars beyond 100 mly as some answers have lacked.



The methods of measuring distance to stars beyond 100 light-years:



Cepheid variable stars. These stars change in brightness over time, which allows astronomers to figure out the true brightness. Comparing the apparent brightness of the star to the true brightness allows the astronomer to calculate the distance to the star.



Stellar motions: All stars are in motion, but only for nearby stars are these motions perceivable. Statistically, therefore, the stars that have larger motions are nearer. By measuring the motions of a large number of stars, we can estimate their average distance from their average motion.



Moving clusters: Clusters of stars travel together, such as the Pleiades or Hyades star clusters. Analyzing the apparent motion of the cluster can give us the distance to it.



Inverse-square law: The apparent brightness of a star depends both on its intrinsic brightness (its luminosity, or how bright it really is) and its distance from us. If we know the luminosity of a star (for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities), we can measure its apparent brightness (also called its apparent magnitude) and work out the distance using the inverse-square law. There are several variations on this, many of which are used to measure distances to stars in other galaxies.



Interstellar lines: The space between stars is not empty, but contains a sparse distribution of gas. Some times this leaves absorption lines in the spectrum we observe from stars beyond the interstellar gas. The further a star is, the more absorption will be observed since the light has passed through more of the interstellar medium.



Period-luminosity relation: Some stars are regular pulsators. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star. If we measure the period of such a star, we calculate its luminosity. From this, and its apparent magnitude, we can calculate the distance.

Answer 8

Through "mile markers" called Cepheid variables. Those are unstable stars that change in luminosity in a very characteristic way. If we see that "flicker" in a distant galaxy we know that it is a Cepheid variable. And since we have studied Cepheids closer to home and know their real (absolute) luminosity we can calculate how far away it would have to be to appear as bright or as dim as it is in the distant galaxy. And since stars outside the Milky Way resides in galaxies we only need to find the distance to that whole galaxy to know the distance to the other stars in it.

Answer 9

Generally, they use "red shift" to determine how far away galaxies (and stars) are. By determining the star type (A Cepheid variable is the same, no matter which galaxy or location it's in), they can look at it's spectrum and see how far the light has shifted to the red. A high shift means it's moving away at high speed, and generally how far away it is.

Answer 10

I believe that its done by wavelength and luminescence of the light.