Hey. Tried searching for this but was unsuccessful.
Now, to preface this, I know absolutely nothing about science. I’ve watched a few documentaries, but that’s as far as my knowledge goes.
I have been wondering about something for a while but whenever I try looking it up, the answers are far too complex for me to understand.
So my question revolves around stars. First, how is distance to stars measured? How do we know that it is reasonably accurate, and not a situation where said star was smaller and closer or bigger and farther away than previously thought?
Second, how is the brightness measured? I’ve heard stuff like “this and that star shines 100x brighter than our sun”. How can that be reasonably accurately measured over the vast distances we’re talking about in space?
Third, how can we measure or calculate mass of a star, given the vast distances?
I suspect those 3 questions might be somewhat linked, so I thought I’d ask all 3 in the same thread.
Now keep in mind, I do not understand calculus or math above algebra, so if anything like that is needed to understand, please dumb it down as much as you reasonably can.
Thankful for any replies helping me wrap my head around these concepts. Thanks in advace!
In: 9
It’s a complex topic for exactly the reasons you brought up, and astronomers have a variety of different approaches depending on what the object is and how far away it is.
Things that are (relatively) close can be measured by parallax. As Earth moves around the sun, their position in the sky (relative to things that are very far away) moves too, and this depends on how far away it is from Earth. (This method also depends on how far Earth is from the sun, which we learned in the 18th century using the transit of Venus).
Moderate distances can be measured with a “standard candle.” An object that we think has a fixed luminosity based on what we know about its physical properties and similar objects within the useful range of parallax. We can then use that fixed luminosity to back out the distance to Earth.
Longer distances use a variety of techniques, collectively called the Cosmic Distance Ladder.
We can calculate the distance to a star by measuring how much it shifts its position in the sky half a year apart. In that time Earth moves from one side of its orbit around the Sun to the opposite side, and that is far enough that stars change their positions noticeably provided you can make precise measurements. Knowing the distance and the visible brightness of a star you can calculate its absolute brightness. Determining mass is less straightforward and involves measuring the speed of motion of stars in binary systems around each other, estimating from correlations from brightness and spectrum, observing gravitational lensing of light etc.
>First, how is distance to stars measured?
For closer stars, we measure their apparent position against the background at different times of the year. Based on the diameter of Earth’s orbit, astronomers can use trigonometry to calculate the star’s position.
>How do we know that it is reasonably accurate, and not a situation where said star was smaller and closer or bigger and farther away than previously thought?
Astronomers use red-shift to calculate distance to farther objects. Since the universe expands at a known rate, objects at certain distances open at particular speeds, which produce certain amounts of redshift.
Knowing the average redshift at particular distances, combined with knowing a star’s composition (see below), and accounting for redshift of its neighbors, scientists can estimate distance and motion of very distant objects.
>Second, how is the brightness measured?
>
Knowing what a star is made of (see below) gives us an idea of its mass and how much light it should emit. Physics very clearly lays out what kind of characteristics a star’s matter composition yields.
>Third, how can we measure or calculate mass of a star, given the vast distances?
>
When hot, matter emits light that is absent specific colours depending on its elemental composition. Every element has its own spectral signature. These signatures persist even if the star’s light is redshifted.
Astronomers use these color signatures to determine what a star is composed of, knowledge physicists then use to determine its mass. Stars of certain elements must be of a certain size to actually exist as stars. Measuring the redshift of a star of known composition gives its distsnce.
Wow. Have read the top comments, and while I am sure you all know what you are talking about, it is still unclear to me. I feel dumber than I did before.
Edit: To be fair, I’m pretty sure this is not a subject that could actually be explained satisfactorily to a 5-year-old. Thanks anyway, smart folks!
Astronomers use an effect called parallax to measure distances to nearby stars. Parallax is the apparent displacement of an object because of a change in the observer’s point of view.
the brightness of these stars using the magnitude scale. The magnitude scale seems a little backwards. The lower the number, the brighter the object is; and the higher the number, the dimmer it is. This scale is logarithmic and set so that every 5 steps up equals a 100 times decrease in brightness.
The formula 𝑀 = 4𝜋²𝑟³/𝐺𝑇² can be used to calculate the mass, 𝑀, of a planet or star given the orbital period, 𝑇, and orbital radius, 𝑟, of an object that is moving along a circular orbit around it.
Hold your finger out in front of you, sticking straight up. Look at it. Now close your right eye. Still see it? Now close your right eye and open your left eye at the same time. See it move? Keep doing that.
Camera one, camera two. Camera one, camera two.
This effect is called “parallax”. The position of the thing you’re looking at seems to move based on the angle you’re seeing it from. Instead of looking at your finger, if you look at something across the street, it won’t appear to move as much. The difference in angle isn’t as big. If you look at something really really far away (like a mountain range, or the Moon), it won’t appear to move at all. This is very obvious if you hold up your finger, and behind that finger you can see the house across the street, and behind that house you can see a mountain range in the background. Now when you do it, your finger seems to move a lot, the house only a little, and the mountain not at all.
We can do the same “camera one, camera two” trick from Earth, when looking at the stars. But to make the angle as big as possible, you need a lot of distance. The easiest way to do that is to wait six months. In June, the Earth is on one side of the Sun. In December, it’s all the way on the other side of the Sun. So you take pictures six months apart, and compare them, and see which stars have moved and by how much.
Stars that are closer to us will appear to move a lot more than stars that are farther away. For closer stars, we can use trigonometry to measure their distance. We know how much they appeared to move, and we know how far away they would need to be to move that same amount. The better your telescopes, the more precise your measurements, and the better your distance calculations can be.
We have done this *a lot*. Lots of people and lots of telescopes, and everybody checking each other’s work to make sure they didn’t screw up. Lots and lots, on every star even kind of close enough to measure.
For distance there is the [Cosmic distance ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder). For relativity nearby stars we can measure how their apparent position changes slightly as we move from side to side, orbiting the sun every year. That’s basic trigonometry so we’re very confident about the principle, but the tiny angles mean accuracy drops off with distance. But some types of stars have a consistent brightness. We can measure this with nearby stars and then extrapolate to more distant stars, though with more uncertainty. Yet more techniques can be used for even more distant objects.
We can use apparent brightness (how bright it looks) to absolute brightness (how bright it should be). For other galaxies, we’ve used variable stars (stars whose brightness varies on a predictable time frame, which is related to absolute brightness). With billions of stars to choose from, some of them have unique qualities that correlate to other properties.
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