Physics questions - stars, time, relativity

This is not doing my homework; I’m teaching Physics, Chem, and AP Enviro Sci at a small high school with no other physical science people here, and my real subject is chemistry. There’s no one here who even understands the questions, so answers are not going to come from the staff. In an effort not to bug my physics major brother into hating me, I’m going to ask some things here. The textbook is really more of an 8th grade book, and I have a lot of high performing 10-12th graders who are much more informed and questioning than the book provides for.

We started the the chapter on relativity, and this started great debates on the nature of time and things like “what was there before the Big Bang.” If you are inclined, please help.

  1. How do we know the Milky Way has arms?

  2. Exactly how do we know that other galaxies are receding from us, and how is the speed measured? I know it’s a red shift in the light, but how do we know the “correct” color of the light?

  3. Are more stars being formed than are dying? Since good telescopes are such a recent invention, how do we know this?

  4. Is our super galaxy cluster receding from other clusters, or is the cluster itself expanding?

  5. How does strong gravity affect time and why?

  6. Light always seems to have the same speed to everyone in every reference point, but how is that speed measured if you don’t know where it started and the time it took to get to you?

  7. Is there any sense of something that the known universe is expanding into?

There are likely to be more. Thanks for any answers!

Uneducated but curious layman here, so take what I say with a few pounds of salt.

  1. Dunno, but we know our galaxy is flat and flat galaxies tend to have arms. I assume there are measurements to confirm the arms.

  2. Red or blue shifting moves the whole spectrum, and since spectra are determined by the atomic components of the stars, we can compare them with the spectra of “static” samples in the laboratory, and see that they correspond with common materials, only shifted.

  3. Dunno.

  4. AFAIK our cluster is receding from other clusters, and I think the only galaxies that are not receding away from us are in the local group (a small subset of our supercluster).

  5. Other people may be able to explain this.

  6. The earth moves around the sun, the sun and the earth move around the center of the galaxy all at very high speeds. Whichever way you’re measuring (north, east, south, west, up down etc), the speed of light remains the same, and we’ve got measurements accurate enough to be able to detect the difference if the speed of our planet did affect the speed of light. Which is one of the things that got Einstein and his collegues going in the early part of the 20th century. ETA: let’s just take the light of the sun: we DO know where that came from. Or we can measure the speed of light of a LASER or whatever. The question of where the light came from isn’t really relevant, as far as I can see - please clarify if this makes no sense.

  7. Dunno. But as far as I can gather, the answer appears to be “there’s nothing the universe is expanding into” or “there’s no way to know”.

Based on guesses from Physics and Astronomy 30 years ago:

  1. I’m guessing by doing star population counts. The big stars (blue giants, O,B,A) are obvious from hundreds of light years away and can be distinguished from nearby dimmer stars by the spectrum characteristics - relative helium to hydrogen, etc. We count the population and notice where there are more or less in our area. Plus red-shift indicates relative motion toward or away from us.

  2. Red shift. Google the key words from below… Elements like hydrogen and helium absorb a particular band of light at a certain frequency. The farther the galaxy based on distance=brightness, the more red-shifted the spectral lines, the faster it is recending, in general.
    There are variable stars (Cephids?) whose period-to-size ratio (hence, absolute brightness) is well documented in the local neighbourhood. Applying that ratio to cephids in not-too-distant galaxies gives a distance to shift value (Hubble constant?) which indicates distance vs. red-shift. Apply that red-shift value to super-distant galaxies to determine average brightness to distance to show on average, the further the galaxies, the faster they recede from us.

  3. Probably. You can find certain nebulae where the stars are still being formed from the coalescing cosmic debris (Horsehead nebula?), but most of that seems to have happened already.

  4. Both, IIRC.

  5. Time slows down the heavier gravity gets IIRC. That trivia of relativity I’ve forgotten.

  6. Don’t care. We measure it as it goes by. No matter what your speed, relative to anything, light measures the same speed as it goes past us; but we can measure the “red shift” of the spectral lines from motion.
    Thus, the classical Micehlson-Morley interferometer shows light from a star for example, travels the same speed whether we measure it as the earth in its orbit goes toward or away from that or any star. The earth goes around the sun at about 7mi/sec, IIRC, so it’s easy to determine whether light travels faster or slower by that much. It’s always the same.
    Similarly, a MM-interferometer will measure the same speed of light as you rotate the instrument on its center. If light were travelling in a hypothetical aether, it would have different speeds indifferent directions.
    Relativity and e=mc^2 are a direct result of a though experiment, simply calculating “what happens to basic physics equations if everyone sees light travelling the same speed”? Well, 2 people travelling at different speeds see the same light from the same A to B as a shorter time and/or distance.

No. It is everything, it’s just getting bigger. It’s a metaphysical question like “what’s outside heaven?”

  1. It’s fairly easy to figure out that the MW is disk-shaped because it forms a narrow band in the sky. All the other disk-shaped galaxies we can see have arms. I believe that there have recently since been galactic surveys that have confirmed that stars are clumped in arm-like bands.

  2. There are a variety of phenomena known as “standard candles”. These are astronomical events that always have about the same level of brightness. For example, certain types of supernova always explode the same way. By measuring how bright the “candle” is, you can tell how far away it is. In reality there are a bunch of different techniques that are used to validate each other.

  3. The spectrum of stars changes in a particular way as they age and their composition changes. Using this info we can identify areas that have lots of new stars. However, compared to the total number of stars, new star formation is fairly rare.

  4. The Milky Way is part of a cluster called The Local Group that includes the Andromeda Galaxy and the Greater and Lesser Magellanic clouds, among others. All the objects in The Local Group are gravitationally bound to each other and the group is not expanding.

  5. That’s General Relativity. Time flows slowly in any accelerating reference frame. So if you’re in a rocket with the engines firing, or standing on the surface of a planet with gravity pulling down on you, time slows down. As to WHY that happens … that’s how the universe is. I’m not sure there’s a better answer than that.

  6. We can confirm the speed of light by emitting a signal, bouncing it off a distant mirror, and measuring how long the pulse takes to come back.

  7. There’s nothing “outside” the universe for it to expand into. Think of the universe as an infinite rubber sheet with galaxies drawn on it. As it stretches it’s still infinite, but everything is farther apart.