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# Questions about measuring distance and temperature in space

Can someone briefly explain to me how they measure both distance (length or width) and temprature in space? For example, you'll see a picture of a nubula and astronomers will say it is something like 40 light years from one end to the other. But how can they measure that accurately from something that is clearly so far away?

Similarly they'll name a star that is 100x larger than our sun with "X" amount of mass, and "X" degrees temperature. Again, how can they measure that kind of stuff?

I understand the doppler affect and how they can judge directional movement either towards or away from us by shifts in light waves, but what if something is moving perpendicular to our line of vision?

I also kind of comprehend how they use light defraction to determine chemical composition of distant objects, but is this how they are determining temperature and mass?

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### Replies to This Discussion

There are several ways of measuring these things, and they are all inter-related. For instance, knowing the brightness of a star can help determine its temperature, but you have to know how far away it is also. The more of these inter-related measurements you make, the more accurate your total picture of the star will be.

For example, to know the width of a celestial body, you need to know its distance from Earth, and the visual angle it makes in the sky. To know the distance from Earth, you can measure red-shift if its a distant galaxy, or parallax if it's a nearby star. You can also compare it to other stars of similar brightness.

To measure mass, you can measure brightness and colour. The colour helps you determine what type of star it is and what stage of its life-cycle it is in. From this you can estimate its mass. More massive stars are brighter and bluer and have shorter life-cycles. Less massive stars are dimmer and redder and have longer life-cycles. If you know the distance, brightness, and colour of a star, you can estimate its mass.

Another great measurement is the spectrograph of the star, which tells you which elements compose the star and in what ratios. This is a further indicator of the type of star and what stage it's at in its life-cycle.

To determine perpendicular motion, you would need to know the distance to the star and measure its changing position in the night sky over a long period of time. We have measurements going back many years, even to the time of Kepler and Galileo if you want to take it that far back.

A good way to develop a strong estimate is to establish a good model of the local universe and identify what role the star plays in that model. For example, if the star is a member of the Milky Way, you can use the model of the Milky Way to help determine its position and relative velocity. If the star is in near our Sun in the Milky Way, it will probably be moving at a very similar velocity to the Sun simply by virtue of the fact that the Milky Way is spinning at a particular speed and all the stars at a particular location will all be moving at the same velocity. If the star is in a more-distant part of the disc of the Milky Way, then it will have a different relative motion simply due to its position in the galaxy and the speed of rotation of the galaxy.

Also, having a good model of the inner workings of stars helps to identify the characteristics of the star from incomplete measurements. For example, we know that stars are basically enormous fusion reactors, and we know the physics of nuclear fusion, so we know that if a star has a certain mass, it will cause fusion at a certain rate, and that will produce so much heat energy, and that heat energy will cause the star to be a certain size and brightness and colour. So, if we measure the brightness and colour, we can infer the size and mass of the star with some accuracy. There are always error-bars on these estimations though. The more accurate measurements you make, the smaller the error-bars of the estimations become.

So, the more information you gather about a star/whatever and its role in its local part of the universe, the more you can tweak and tune your model of it to make an accurate estimation of everything from velocity, to mass, to temperature, to physical composition, to life-cycle stage, to age, etc. etc. Most of these measurements are all inter-related and so each more accurate measurement sheds extra light onto the other physical characteristics of the celestial body.
Thank you for that.

To know the distance from Earth, you can measure red-shift if its a distant galaxy

So this is how they would determine if a star was 50 million light years away from us, or 75 million light years away from us? When dealing with that kind of distance, how could they know they were accurate, and is there a window of possibilities/variations?
Like I said, each measurement has a particular accuracy, which leads to error-bars in the various estimates. So you might get an answer of 50 +-10 million light years, which means it could be 40-60 million light years. As you gather more measurements you can narrow the error bars, so you might use a separate measurement to narrow it down to 55 +-1 million ly, or whatever. The more info you gather, the more accurate the estimates.
Thank you. I appreciate you taking the time to clear this up for me.
Thanks Wonder - good explanations - especially for us non-mathematicians.
This is a more mathematical outlook, but even if you aren't a math wizard this may help as well.
http://en.wikipedia.org/wiki/Parsec

Basically they used trig and measured the change in a stars placement in the sky versus the Earth's orbit. We can get the actual distance of the orbit by how long a year takes and Kepler's laws and such. With that, we can convert the parsec into real distance.
Thanks. I looked at that, but that is over my head. I can't even remember how to divide fractions. :)
Dallas, if you're really interested in astronomy topics like this, I highly recommend the AstronomyCast podcast.
Thank you for that. I like it already.

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