Protokoll

Astrophysik1
Prof. Refregier, HS 2013
Zusammenfassung Im schönen Eckbüro von Prof. Refregier im HIT mit seinem Assistenten. Erst
werde ich aufgefordert mich hinzusetzen, am Ende steh ich dann aber doch um an der Tafel Sachen zu
erklären. Sowohl Assistent als auch Professor stellen Fragen. Draussen wechseln sich Schnee und Sonne
ab (woraufhin Prof. Refregier jeweils aufspringt und die Jalousien auf- bzw. zumacht).
Ablauf Prof: Hello, welcome, please sit down. We drew three topics randomly like we announced in
the lecture. So let’s start with the first one: What can you tell us about globular clusters.
Ich: A glob. cluster is a group of tightly bound stars. Which is interesting about them, is the approximate
same age and distance from us. That makes it possible for us to determine both of these parameters
from HR diagrams. Also we find them mostly in the galaxy’s center, so like this we can find out where
this center is and determine the distance to it.
Prof: Do we really find them in the center?
Ich: Well I think that’s how we find out where the galactic center is. But there are also some distributed
in the halo and bulge.
Prof: Yes that’s it, they’re mostly in the bulge and halo.
Ich: Ok so they are around the galactic center...
Prof: Is there more you want to tell us about glob. clusters?
Ich: There are around 105 stars in a globular cluster which make them useful for determining parameters
for example from the spectrum.
Prof: What about gravity?
Ich: They’re stable, so neither growing or shrinking, so gravity has to be balanced, which is in this case
done by random motion.
Prof: Good. You mentioned that you can find out something with the HR diagram. Can you explain
how that works?
Ich: Explain on the whiteboard that the main sequence of nearby stars is the same as for a globular
cluster, so we can fit it and from that determine the absolute magnitude. Also from the turning point of
the sequence of the glob. cluster we can find out the age from τ = M −2 (figure 1).
Prof: Yes very good. Do you know what for this was also important historically?
1
MSc Physik, Wahlfach
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Abbildung 1: HR-diagram of nearby stars.
Ich: It’s possible to put a constraint on the age of the universe, because it can’t be older than the
oldest globular cluster.
Prof: Right. Was that a problem? There was a big discussion about that...
Ich: Well... not sure what he wanted.
Prof: What other ways are there to determine the age of the universe?
Ich: Quite a lot I’d say, but I don’t know exactly what was historically used... hesitating
Prof: Ok but from cosmology...
Ich: Yes from Hubble’s law, the inverse of the constant, from the expansion of the universe we can get
it.
Prof: Good. Do you know the value for this? Actually these questions about historical things and age
was the question of the assistant. At this point he asks the prof it this was covered in the lecture. Not
really.
Ich: Well probably it was bigger than the present one, so the inverse, the time, is smaller than the
oldest age of globular clusters.
Prof: Yes, that’s right. So let’s come to the next topic, a bit larger scales. What kind of galaxies are
there.
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Ich: First there are normal and active...
Prof: Interrupts me. Yes for now we just consider normal galaxies.
Ich: Then there are spirals and ellipticals. Spirals have circular motion, have ISM and therefore also
new stars are born there in contrast to ellipticals.
Prof: What about their age?
Ich: Ellipticals have older stars than spirals, so they appear redder to us. Also spirals turn into ellipticals
by loosing their ISM, so if we look to further distances we see more spirals than there are around us.
Prof: Well there’s actually a lot of recent research going on there. Can you tell us more about their
shape?
Ich: Ellipticals are ... elliptical smile then for spirals there can be some without bar or with bar and
then there are also irregular ones which are neither of these.
Prof: How does the surface brightness profile look of ellipticals and spirals?
Ich: That I didn’t know... Well I’m not sure about this. Ellipticals are more spherical, spirals more flat,
so I would think that the brightness falls off less for ellipticals.
Prof: Makes some signs with his hands Do you know what Vaucouleurs law is?
Ich: Well I remember the name, but I don’t know the exact form of it.
Prof: Ok qualitatively you said it right, it’s just exponential but different rate of decay for both galaxies.
So next we have a look at fluid dynamic, which is used a lot in astrophysics. We had these equations...
Do you know what they were about? Question was very cautious, maybe they thought I didn’t know hehe
Ich: There are three, the first one is continuity equation ∂t ρ + ∇(ρv) = 0. It just means that the
amount of material which changes inside a volume has to flow in or out through its surface. It follows
from considering a box and using Gauss law. The second one is the Euler equation (write it down). So
it says that the velocity is equal to...
Prof: Is it the velocity?
Ich: Ok no of course not, it’s the change of the velocity, so the a- or deceleration. On the right side we
basically have the force, once acting on the surface and second acting on all points inside the fluid.
Prof: That’s right. What exactly does this derivative mean?
Ich: Uhh that’s quite mathematical, it’s the Euler derivative, which is the total derivative in contrast
to the partial.
Prof: Yes but what does it mean physically? I don’t know this, after same back and forth when they see
that I’ll not get there, the assistant explains: The Euler derivative is the change when you move with
the fluid, the partial derivative is when you are fixed at one point and look at the change there. Ok now
I remembered, damn.
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Prof: And the third?
Ich: That’s just the derivative of P/ργ is zero. But we didn’t really motivate this equation. It just
follows from an ideal gas where this quotient is constant, so the derivative is zero.
Prof: Yes we didn’t really motivate it. It anyway can be any equation, but why do we need this?
Ich: Because we have three variables ρ, P and v so we need three equations to solve them.
Prof: Good. The force on the right side, how can we also write it?
Ich: What is important is, that it’s the force per unit mass. So maybe we can write it as
GM
.
r2
Prof: But for a fluid we don’t have single particles with masses, so how else could we write this?
Ich: We could put the density times the volume instead of the mass.
Prof: There’s another way. If we have a potential... now I know what he wants
Ich: Of course, you can write it as minus gradient of the potential.
Prof: That’s it. Ok but now we have another variable we don’t know. How else can we express it from
the variables we have? I hesitate, so the prof helps: Poisson...
Ich: Of course we can write it as Poisson equation ∆Φ = 4πGρ.
Prof: Very good. Ok thank you, we’re done.
Ich: What, already?
Bemerkungen Das erste mal bei meinen Prüfungen, dass auch der Assistent gleichberechtigt mit
dem Prof gefragt hat. War aber deshalb auch sehr relaxte Atmosphäre, sowohl Prof als auch Assistent
sind sehr nett und unterstützend. Manchmal fast schon ein bisschen viel, haben fast schon zu schnell
weitergeholfen und mir wäre manches vielleicht eingefallen. Aber unterm Strich sehr angenehm wie sie
es machen. Herleitungen braucht man nur sehr wenig, man muss aber auf jeden Fall wissen, was die
Gleichungen bedeuten! Etwas überraschend, dass die Prüfung schon nach 25 min fertig war, aber ich
war wahrscheinlich nicht der einzige nachdem wir schon 15 min früher angefangen hatten, weil sie beim
Student davor auch so früh fertig waren. Auf jeden Fall cooler Prof für mündliche Prüfung.
Erwartete Note: 5.5
Erhaltene Note: 5.25
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