Edition 16 of the Astronomy On-line Newspaper

An Answer to Some Questions

We are forwarding the letter with questions sent by our CYG Group to several professional astronomers and the wonderful answers which we received from Mr. Hauke Fiedler from the Institut fuer Astronomie und Astrophysik, Muenchen, Germany. We especially like the answer to question No. 4. It is really fine! We would be happy if the message we got is published in Astronomy On-Line. Here are first our message and then Mr. Fiedler's answer:

Dear Sirs,

We are the CYG Group (Curious Young Geniuses) from Varna, Bulgaria. We attend the astronomical courses lead in the Astronomical Observatory & Planetarium "N.Copernicus". We would be greatful to you if you spare us a bit of your time and send us your answers of some of our questions.

Thank you in advance!

Sincerely yours, the CYG Group, Varna, Bulgaria.

e-mail: astro@ms3.tu-varna.acad.bg

Hi folks,
now I am trying to answer your questions as well as I can.

>       I. Questions asked by Galin Genchev, 17 years old:
> 1. How the parallax of a star is measured in order to determine the
> distance to the star?
During every year a nearby star describes relatuvey to the background
stars an ellipse (on the pole of the ecliptic it is a circle, on the
equator of the ecliptic it is a straight line). Now, 1'' equals 1 parsec
= 360*60*60/2/pi astr. units. The astr. unit is the distance from the
Earth to the Sun. From the ellipse you have the angle, and now you can
calculate the distance. You can do this till max. 50 parsec. The first
distance measured applying this method was 61 Cygni, p=0.293''

> 2. How astronomers measure the luminosity of the stars?
This is not very easy. First of all you need the bolometric brightness
m_bol. This includes all radiation from this star (of course outside the
atmosphere without any extinction). The equation to calculate it is
m_bol = -2.5*log pi*R*R*F/r/r + constant1
whereas
pi * F = sigma T_eff^4
The constants:
R = radius of the star
r = distance to the star
F*pi = total radiation of the star including absorption and emission
sigma = Boltzmann-const.
T_eff = effective temperature of the star.
The constant1 normally is defined like this: At the temperature of the
sun the bolometric correction equals zero:
B.C. = m_bol - m_v
whereas m_v is the visual brightness.
Now you have the bolometric brightness and you are able to calculate the
absolute bolometric brightness M_bol. Therefore you have to set your star
in the distance of 10 parsec and you can calculate
m_bol - M_bol = 5*log(r/10)
Finally, you have the formula connecting the absolute bolometric
brightness with your luminosity:
M_bol - 4.72 = -2.5 log(L/L_sun)
with L_sun = 3.82 10^33 erg/sec

> 3. It is known that the trajectory of the Sun around the galactic
> centre is not flat, the Sun plunges and emerges above the galactic
> plane like a dolphin with a period of about 25 million years. How far
> from the galactic plane does it go?
The orbit of the sun lies always within the galactic plane, although
there are changes in galactic latitude.

>       II. Questions asked by Denis Mehmedov and Elena Sarbinska, 12 years old,
>       and Srebrina, 10 years old:
> 1. Do the black holes emit radio signals?
A black hole does not emit anything. What you have on the edge of the
Schwartzschild radius is 'paarerzeugung' - an electron and a positron is
born. Now, one of these two particles might fly back inside the black hole,
the other to the outside. With this process the black hole can loose mass
by radiation. The particle outside can produce any radiation - radio or
optical, whatever you want. But, the more likely process is that you find
material on a disk surrounding the black hole. These material spirals
into the black hole and does emit radiation, which you can observe from
the Earth.

> 2. How would the starry sky look like on a planet orbiting the farthest
> star from the centre of our Galaxy?
I don't know. But it would be totally different to our starry sky - most
of the stars you watch here are to distant to be observed by eye. The
milky way would shine only over one side of the starry sky - the other
side would be black, maybe some other galaxies visible, due to the lack
of stars. Remember, you are at the edge of our galaxy.

> 3. How will the orbital motion of the planets change when the Sun
> becomes a red giant? What will happen then to the life on the Earth?
When the Sun will grow, the angular momentum will change. Therefore the
inner planets will fall into the sun. But this will last a couple of
billion years. What is more likely. in about 2.4 billion years the Earth
is outside the hability zone - due to the expansion of our sun. Then the
temperature on our planet would not be very convenient. All bigger life
forms like animals will probably die.

> 4. Is there in the Universe a kind of matter that resists the gravity?
Yes there is: Love, but nothing else.

> 5. Have the asteroids gravity?
Of course, due to their mass. Gravity is defined by Newton
F = -G * m1 * m2 / r^2
m1, m2 are the two masses, G is the general gravity constant, and r their
respective distance.

> 6. Why men still haven't visited Mars?
It would take a human about 1.5 years to visit Mars and return. The stay
would last only about 10 days, otherwise you have to wait to long to fly
back. this is due to the constellation of Mars and the Earth. Other
problems that exist include muscle loss in space, you need lots of energy
for life supporting systems, and every kilogram accelerated costs energy.

> 7. Who created the largest (10 m) optical telescope? Which was the
> first object observed by it?
Since the last three years there are some 10-m telescopes:
- The HET in Texas, USA, has an effective area of about 11m
- At Mauna Kea, Hawaii, there are two: KECK 1 and KECK 2 from the USA
with an effective area of 10m.
The first object observed? I don't know. But maybe you can ask some
astonomers in the USA/Hawaii.

> 8. Why was the Universe created?
This is more a philosophic question. Maybe you try to ask a preacher or a
priest.

> 9. Is it possible a planet to revolve around a black hole?
Yes, why not? There are even double stars which one is believed a black
hole, e.g. Cyg X-1 (That is in Cygnus (Cyg) the brightest (1) x-ray (X)
source).

> 10. Why the Earth doesn't fall on the Sun? If there is no perpetual
> motion why the Earth doesn't stop moving?
You have a equilibrium of gravity and centrifugal-force. If you slow down
the motion of the Earth, it will orbit the sun on closer circles.

> 11. Why there is no water in the Moon's seas?
The moon could never keep an atmosphere - it is not heavy enough.
Therefore there could not be any water on the moon, every drop would have
evaporated.

> 12. How the Saturn's rings formed?
Once there was probably a moon revolving Saturn. This moon was to close
to Saturn and bursted. What you see is the remnants of the moon
surrounding Saturn due to Kepler's laws.

I hope these answers will satisfy you. If you have additional questions
feel free to ask - don't be shy.
Best to you,
Hauke


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	                     DON'T  PANIC

Institut fuer Astronomie und Astrophysik

Hauke Fiedler                      email: hauke@usm.uni-muenchen.de
Scheinerstr. 1
81679 Muenchen                     tel. 0049 - 89 - 9220 94 - 49
Germany                                                     - 48
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	                        -\_<,
	                      (*)/'(*)
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Best wishes,

Eva Bojorova, Varna, Bulgaria

Richard West
ESO Education and Public Relations Dept.


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Last update: Nov 28, 1996