Astronomy Q&A game

[Ivan Seeking]

Originally posted by schwarzchildradius
only if you consider light abstract, which I guess it kind of is. if you could "ride" light, of course, you wouldn't be moving through time.

"...one can affect his position in time but not in space"

No voodoo needed. But it wouldn't be pretty! schwarzchildradius

 

[J-Man]

Originally posted by Ivan Seeking
The language used may be a little optimistic:
What are the hypothetical circumstances, if we can show [mathematically] that one can affect his position in time but not in space?

I'm not sure I understand the question but...

f(x) = 0 * x = 0 for all x
f(y) = 0 * y = 0 for all y
f(z) = 0 * z = 0 for all z
f(t) = h(t)

Where h(t) is some function of time independent of the 3 spatial "variables".
Basically, position is invariant, or the coordinate system used moves with the hypothetical person.

 

[Ivan Seeking]

Originally posted by J-Man
I'm not sure I understand the question but...

f(x) = 0 * x = 0 for all x
f(y) = 0 * y = 0 for all y
f(z) = 0 * z = 0 for all z
f(t) = h(t)

Where h(t) is some function of time independent of the 3 spatial "variables".
Basically, position is invariant, or the coordinate system used moves with the hypothetical person.

Clever, but I don't mean to be that tricky. I am trying to ask a hard question not a tricky one; but the thing that makes it hard is tricky.

Hint: Where is our schwarzchildradius?

 

[marcus]

as soon as one gets inside the event horizon of a black hole

"one can no more avoid the singularity than one can avoid
next Tuesday"

the direction towards the center becomes the time direction.

perhaps this responds appropriately to the question?

 

[chroot]

Originally posted by marcus
the direction towards the center becomes the time direction.

I can't say I agree with the either this message or the language itself.

It is true that all worldlines inside a black hole at some point intersect the singularity, but it is not true that all worldlines are geodesics. You can still have fuel in your rocket when you cross the horizon, and you can still zoom about inside the horizon in non-inertial motion.

The bottom line is that since two observers can cross the horizon at the same place and follow two different trajectories once inside, there's no way you could even qualitatively think of "the direction towards the center" as being the "time direction."

I think this question is bordering on the philosophical, since it eventually comes down to semantics.

- Warren

 

[schwarzchildradius]

Ah, could it be that Ivan is seeking the conditions on the inside of a black hole event horizon?

 

[Ivan Seeking]

Originally posted by marcus
as soon as one gets inside the event horizon of a black hole

"one can no more avoid the singularity than one can avoid
next Tuesday"

the direction towards the center becomes the time direction.

perhaps this responds appropriately to the question?

I was wondering how many times I could say schwarzchildradius and get away with it. CORRECT!
You're up marcus.

 

[Ivan Seeking]

Originally posted by chroot
You can still have fuel in your rocket when you cross the horizon, and you can still zoom about inside the horizon in non-inertial motion.

The space coordinates exchange roles with time and become imaginary. According to my notes from a senior level GR class - taught by a very good professor - this is the proper interpretation. If this is incorrect or outdated, I am not in a position to argue this point with much proficiency.

 

[marcus]

Originally posted by Ivan Seeking
You're up marcus.

The first star to which the distance was measured is_______

(I don't mean the sun I mean a "real" star.)

Who measured the distance? In what year?
How far away is this star?

For extra points, what is the parallax angle by which
the first stellar distance was determined?


Footnote---not part of question---Christian Huygens estimated the distance to the star Sirius in a clever way long before parallax measurement was possible. How he did it is almost funny. But I am not considering that as the first real measurement, so it would
just be distracting to describe his method.

 

[Labguy]

Originally posted by marcus
The first star to which the distance was measured is_______

(I don't mean the sun I mean a "real" star.)

Who measured the distance? In what year?
How far away is this star?

For extra points, what is the parallax angle by which
the first stellar distance was determined?


Footnote---not part of question---Christian Huygens estimated the distance to the star Sirius in a clever way long before parallax measurement was possible. How he did it is almost funny. But I am not considering that as the first real measurement, so it would
just be distracting to describe his method.

(A) 61 Cygni, a double of red dwarfs.
(B) F.W. Bessel in 1838.
(C) 10.3 LY.
(D) 0.29 arc seconds.
(E) Measured by parallax.

 

[marcus]

Originally posted by Labguy
(A) 61 Cygni, a double of red dwarfs.
(B) F.W. Bessel in 1838.
(C) 10.3 LY.
(D) 0.29 arc seconds.
(E) Measured by parallax.

Right on! Labguy it is your go.
This is the kind of conversation!
When people know about Bessel, I mean. 1838 a time of giants.
Darwin was just getting Evolution theory written down that year. Faraday visualizing lines of force.
God bless Bessel, put the first yardstick to the stars.
BTW Britannica say he measured 0.31 arc seconds but
so close and don't know who is right you or Britannica,
so call it 0.29.
Bravo. your turn.

 

[schwarzchildradius]

sun was discovered before 1838. Aristarchus, wasn't it? 300 BC?

 

[marcus]

Originally posted by schwarzchildradius
sun was discovered before 1838. Aristarchus, wasn't it? 300 BC?

the sun was ruled out in the question---we just had a question on the thread relating to Aristarchus----so when I asked the question I said the sun was not the answer I was looking for.
better luck next time

Labguy's turn

 

[Labguy]

Ok, another easy one: An easy internet look-up.

Re-list, in order of size (Stellar Masses), from biggest to smallest, the following stellar classes with approximate masses and lifetimes. This is based on a total lifetime of our Sun as 10 billion years. Not restricted to just main sequence.

Stellar Class:
A:
F:
B:
MO:
BO:
K0:
G2 (Our Sun)
O3:
M7-8

Just type a list (in order) with class, ~Mass and ~Lifetime in years (Use millions, billions or trillions).

EDIT: I removed "size" typed in by mistake. The first criteria is mass.

 

[chroot]

I'm not sure this question is well posed.

The order of surface temperatures is OBAFGKM in order of highest to lowest.

However, the spectral type alone is not enough to determine mass. You would also need luminosity. There's no such thing as an "average mass" for a given spectral type.

Lifetime is, of course, tied to mass.

- Warren

 

[Labguy]

Originally posted by chroot
I'm not sure this question is well posed.

The order of surface temperatures is OBAFGKM in order of highest to lowest.

However, the spectral type alone is not enough to determine mass. You would also need luminosity. There's no such thing as an "average mass" for a given spectral type.

Lifetime is, of course, tied to mass.

- Warren

The surface temperatures can only be seen from the spectra of the photosphere. While on the main sequence, the larger mass leads to higher temperatures, so there is a direct correlation between temperature and mass until the main sequence is left behind. There is a definite "average" mass for an M type star vs. a star formed as a B type. Also there is a huge difference in "average lifetimes", as you say dependent on mass. Part of a giveaway is that there is no way that an M type star (depending on whether M0 to M9) will have a mass below 0.08 Sm or much above 0.1 Sm. This type of star cannot create a high surface temperature simply because there is not enough mass to generate the fusion processes that would lead to high temperatures.

The original question stands, but nobody has to be very exacting, just good estimates in correct order.

 

[marcus]

Originally posted by Labguy
Ok, another easy one: An easy internet look-up.

Re-list, in order of size (Stellar Masses), from biggest to smallest, the following stellar classes with approximate masses and lifetimes. This is based on a total lifetime of our Sun as 10 billion years. Not restricted to just main sequence.

Stellar Class:
A:
F:
B:
MO:
BO:
K0:
G2 (Our Sun)
O3:
M7-8

Just type a list (in order) with class, ~Mass and ~Lifetime in years (Use millions, billions or trillions).

EDIT: I removed "size" typed in by mistake. The first criteria is mass.

http://nrumiano.free.fr/Estars/classes.html

You originally wanted the radius, mass, and lifetime of the different spectral classes listed. I found a table on the web that gives figures for these and other characteristics:
spectral type, luminosity, mass, radius, lifetime, surface temperature, and the relative abundance in our galaxy.

No table can be perfect. There is variation which a simple table cannot adequately cover. But we can try to do the best we can by way of giving representative benchmarks of each range.

I don't see any harm in including the radius (that you edited out).
My problem is that I don't know how to copy a table into PF.
It will take time

 

[marcus]

From the site I gave the link to:

Spectral class
Mass (solar mass)
Radius (solar radius)
Luminosity
Surface temperature (degrees K)
Life time (million of years)

W ---- >40---- 20 ---1.000.000 --- 50.000 ----- <1
O5 ----- 32---- 18 -----600.000 ---- 40.000 ----- 1
B0 ----- 16 -----7.4 ----- 16.000 ---- 28.000 ----- 10
B5 -----6.5---- 3.8 -----600 ------- 15.500 -----100
A0 ----- 3.2----- 2.5----- 60 -------- 9.900 ------500
A5 ----- 2.1 -----1.7 ----20 --------- 8.500 ----- 1.000
F0 ----- 1.75 ---- 1.4 ----- 6 ---------7.400------ 2.000
F5 -----1.25 ------1.2 -----3 -----------6.600 ----- 4.000
G0 -----1.06 ----- 1.1 ----- 1.3 --------6.000 ----- 10.000
G2 Sun -- 1 ----- 1------- 1---------- 5.800 -------- 12.000
G5 ----- 0.92 ---- 0.9 -----0.8 ------- 5.500 --------15.000
K0 ----- 0.80 ---- 0.8 ------ 0.4 ------ 4.900 ------- 20.000
K5 ----- 0.69 ---- 0.7 ------ 0.1 ------ 4.100 ------- 30.000
M0 ----- 0.48 ----0.6 ------ 0.02 ---- 3.500 ------75.000
M5 ----- 0.20 ---- 0.3 ----- 0.001 ---- 2.800 ----- 200.000

 

[Labguy]

Very good summary table. It is not exactly the one I had which showed a few more specific examples, but is close enough to be called correct.

I will "cut and paste" part of where I was looking, but don't want to give the URL now because it has lots of neat stuff I might want to hit you with later...

Here is part of my question source:

Type O3: 120 Sm 63 thousand years.
Type O: 25 Sm 3.2 million years.
Type B: 10 Sm 32 million years.
Type A: 2.5 Sm 1.0 billion years.
Type F: 1.3 Sm 5.2 billion years.
Type G2:(Sun) 1.0 Sm 10 billion years.
Type K0: 0.7 Sm 24 billion years.
Type M0: 0.5 Sm 57 billion years.
Type M7-8 <0.1 Sm 3.2 trillion years.

Note the mere 63 thousand years for an O3! Also, other readers should note from your site that class and mass are related. The exceptions come when a star leaves the main sequence. For example, a large, massive red giant swells to where the temperature is very low (red spectrum), while a tiny white dwarf has little mass but is hotter than hell; high temperature.

Your question next.

 

[marcus]

Originally posted by Labguy
Your question next.

In cosmology there is the idea of being at rest with respect to the CMB or the "Hubble flow"----which is the expansion of space.
So there is an absolute rest frame (unlike in special relativity) which corresponds to being at rest with respect to the expansion of space.

The solar system has an absolute velocity with respect to the CMB.
In what direction is it?
What is the solar system's speed in kilometers per second?

 

[chroot]

Labguy,

LOL.. your question specifically stated: "Not restricted to just main sequence."

As stated, it cannot be answered.

- Warren

 

[chroot]

Originally posted by marcus
The solar system has an absolute velocity with respect to the CMB.
In what direction is it?
What is the solar system's speed in kilometers per second?

600 km/s roughly in the Leo/Virgo direction.

- Warren

 

[marcus]

Originally posted by chroot
600 km/s roughly in the Leo/Virgo direction.

- Warren

Virgo and Leo are not in the same direction.
The coordinates of the dipole have been published
but I'd be happy if you would just say in which constellation!

If you can't say in which constellation then pleas give coordinates.

Anybody else have a guess about the km/s speed?

 

[chroot]

Originally posted by marcus
Virgo and Leo are not in the same direction.

Virgo and Leo share a border.

627 +/- 22 km/s in the direction of (l,b) = (276 +/- 3, 33 +/- 3).

Source: WMAP http://www.arxiv.org/abs/astro-ph/0210165

This seems to put it (just barely) inside the boundaries of the constallation Crater, which shares borders with both Virgo and Leo.

Sure seems I was pretty close to being right the first time, Marcus.

- Warren

 

[marcus]

Hi chroot, I am very familiar with this "Crater, 600 km/s" velocity vector because I calculated it for myself back in the early 90s as an answer to another question.

I do not doubt that the paper you reference has that vector in it as one of the results. thanks for the link, which will be useful!

However I suspect you may have misread the paper you cited and not have answered the question I asked. So I will have to
double check and get back to you.

Bravo for getting coordinates!

Originally posted by chroot
Virgo and Leo share a border.

627 +/- 22 km/s in the direction of (l,b) = (276 +/- 3, 33 +/- 3).

Source: WMAP http://www.arxiv.org/abs/astro-ph/0210165

This seems to put it (just barely) inside the boundaries of the constallation Crater, which shares borders with both Virgo and Leo.

Sure seems I was pretty close to being right the first time, Marcus.

- Warren

 

[Labguy]

chroot:

LOL.. your question specifically stated: "Not restricted to just main sequence."

Yes it did, but that followed the statement regarding sun's age of 10 billion years. I should have been more specific as to all the requirements.

 

[marcus]

Yes I was right.
I printed out the paper you referenced
You made a mistaken interpretation of the paper
Your answer is off by on the order of 100 kilometers/second
and, I estimate, several tens of degrees
The hotspot is not in the constellation Crater

(although the coordinates you gave are I believe in Crater, as you say)

 

[marcus]

Originally posted by Labguy
chroot: Yes it did, but that followed the statement regarding sun's age of 10 billion years. I should have been more specific as to all the requirements.

Hey Labguy, do you happen to know which direction (which constellation) the microwave hotspot is in? chroot is way off as to the direction and also the solarsystem's speed relative to the CMB.

If you don't, no problem, but I kind of thought you might.

 

[Labguy]

Originally posted by marcus
Hey Labguy, do you happen to know which direction (which constellation) the microwave hotspot is in? chroot is way off as to the direction and also the solarsystem's speed relative to the CMB.

If you don't, no problem, but I kind of thought you might.

Maybe.., but it should be someone else's turn.

 

[chroot]

All I can think of is that the solar system's velocity is different from the local group's. This makes sense, but who the hell cares about the difference?

The paper I cited is from WMAP's data, which is, of course, a satellite with approximately the same orbit as the Earth. The WMAP figure ought to be valid for the solar system. Or did the WMAP team modify this figure specifically to disclude the solar system's motion so as to get the local group's motion?

This all seems rather semantic.

- Warren