Eli5 How do we measure the distance and magnitude of deep sky objects?
In: 1
We can measure distances in several different ways, but a simple one to explain is “parallax measurement”, where we take one picture in June, and another one in January, compare the difference in position of the star, and do some trigonometry to deduce the distance.
Magnitude is easier and harder, because there are two types. Absolute magnitude and relative magnitude. Relative magnitude is incredibly easy to measure, as it is directly what we measure in a telescope. Absolute magnitude is how bright a star is from some fixed distance. This is harder to see, because not all stars are that distance away from us. Most are much farther.
What we have to do is get measurements of distance first, and we can use the distance and the relationship between distance and relative magnitude to extrapolate absolute magnitude.
The farther away something is in space, the faster it’s moving away from us. We can find out how fast it’s moving by looking at the light from it and seeing how much that light has been red shifted. (Think of red shift as the visual equivalent of how sound changes pitch if it moves past you really fast.)
There’s a simple equation that lets us relate this speed to distance. Using the speed we got from red shift, we can get a good estimate of the distance.
Since most deep sky objects are entire galaxies, we can compare how bright they look to other known galaxies to get magnitude.
Three main methods that I am aware of:
1) Parallax: This is a measure of how much the background moves based upon a small change of observational direction/position. If an object is closer than you and you move a little, you will see a bigger movement of the object relative to the background. The farther the object the less the relative movement. You can check this by holding a finger close to you eyes and moving your head left or right a little bit. The background will appear to have moved a great deal relative to your finger. Now hold the finger as far as you can and make the same movement. The background will have moved much less. Measuring the distances moved and the apparent movement in the background and you can calculate the distance of the object. This works best for stars that are relatively close to us.
2) Standard Candle: When a white dwarf is near a large star it will steal some of the large star’s material. When it has stolen a certain amount it will blow off the acquired material in a nova. The white dwarf will start stealing material again until it explodes again. It does this repeatedly. This explosion releases a relatively constant amount of energy. Since we know how much energy is released we can calculate the distance to that star by looking at the brightness we see from here. The brighter it is the closer it is. They call this a standard candle because it is like measuring the brightness of a candle from different distance using a candle of the same materials and sizes.
3) Redshift: Gases absorb specific frequencies of light when light passes through them due to the arrangement of their electrons. These appear as dark lines in a spectrum of the received light and are sort of a fingerprint for that element. The universe is expanding, so most points are going away from us. Imagine you are the surface of the balloon as it is being blown up. Every point on the balloon is moving away from you. The further away on the balloon the faster the point will be going away from you. Astronomers have worked out a relationship for how fast something is going away from us given the distance to the object. There is a doppler shift for light, just like there is for sound. An object moving towards you will produce a higher sound which for light equates to a bluer color. An object moving away from you will produce a lower sound which for light equates to a redder color. When we look at the light from distant stars we will see that those fingerprints are not where we expected, instead those absorption lines will be shifted to the red side of the spectrum (longer wavelength). We can measure the spectral shift from where those lines should be to where we see them to see the amount of red shift, and from this calculate how far they are. This is why the JSWT was designed as an infrared telescope and not a visible light telescope. It is looking at things so far away that they are moving away so fast that they are shifted outside of the visible spectrum.
Latest Answers