From: Dani Eder <ederd@worldnet.att.net>
Newsgroups: sci.space.tech
Subject: Re: distance measurement..
Date: Mon, 10 Nov 1997 18:56:43 GMT

Richard Ozenbaugh wrote:

> Does anyone have an idea how distances to nearby stars and galaxies
> are
> calculated?
>
> any information would be helpful!!


To nearby stars:  hold up a finger in front of your face, and look at the
finger andwhatever wall is behind it first with only the left eye open,
then with only the right eye open.  The apparent shift of your finger is
caused by the separation between your eyes.  This makes your line of
sight slightly different for each eye to through your finger to the
background wall.  Now replace your eyes with the Earth at two points in
it's orbit 6 months apart, your finger with a nearby star, and the wall
with further away background stars.  The nearby star will appear to shift
with respect to the background stars.  The amount of the shift tells you
how far away the star is.  It turns out the nearest star (Alpha Centauri)
has a shift of about 3/4 of an arc-second.  An arc-second is 1/3600 of a
degree of arc.

With ground-based telescopes you can measure out to about 100 light years
with this method.

To nearby galaxies: there is a certain type of star called a "Cepheid
Variable" whose brightness fluctuates with a period on the order of days.
The period is closely related to the absolute brightness of the star.  If
you know the absolute brightness, you can determine the distance from
that and the apparent brightness (i.e. how bright it appears to us).  A
major objective of the Hubble Space Telescope is to establish the
distances to more distant galaxies by observing Cepheid variables in them
that could not be seen by ground-based telescopes.

The two distance measurements are related, since parallax is used to get
the distance to some nearby Cepheids.

Dani Eder

Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Stars are Suns
Date: Thu, 20 Nov 1997 12:56:49 GMT

In article <650jb4$q0o$2@netnews.upenn.edu>,
Wass <wassman@upenn5.hep.upenn.edu> wrote:
>	He wanted to know when astronomers first KNEW that other
>stars were actually suns, much like our own, just very far away.
>	It seems that there has been much conjecture about this for
>centuries, but does anybody know when the first real evidence came,
>or when this notion was commonly accepted?

You have to distinguish these two criteria from each other.  The idea was
commonly accepted (at least among astronomers!) long before definitive
proof was available.

>	The best answer we have come up with (from a dinner
>meeting with Astrophysicist Vera Rubin and the astronomy faculty of
>Univ. of Penn), is when spectroscopy was introduced...

Not a bad guess, but it obviously came from an astrophysicist. :-)
Spectroscopy appeared in the 1860s.  If there had been an astrometry
specialist in the dinner group, he'd have pointed out that Bessel
published the first measurement of the distance to a star in 1838.
That was what settled it.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Stars are Suns
Date: Fri, 21 Nov 1997 15:48:40 GMT

In article <ant210006313M+4%@gnelson.demon.co.uk>,
Graham Nelson  <graham@gnelson.demon.co.uk> wrote:
>I believe Newton published some discussion to the effect that, if
>the stars were anywhere at all nearby, they would be observed to
>exert a gravitational effect on the orbits of the planets.  This
>at least placed the stars substantially beyond the solar system.

Unless they were relatively small objects whose brightness was somehow
out of proportion to their size and mass.  It was not until stellar
distances were actually measured that anyone could be certain that the
stars really were suns.

Actually, the fact that the stars were well beyond the solar system was
clear before Newton:  the lack of easily-observed stellar parallax got
cited as an argument against heliocentric cosmology, as I recall.  The
apparent retrograde motion of the outer planets near opposition is due (in
a heliocentric model) to parallax, so solar-system objects showed parallax
quite blatantly, but the stars showed none at all.  So either the stars
were centered on the Earth, or they were immensely far away, much *much*
farther away than the planets.

Stellar parallaxes are small even for nearby stars, and it didn't help
that there was no simple way of figuring out which stars are nearby --
many of the brightest stars are not, as Bessel and others doing early
stellar-parallax work found out.  And even the nearest stars are thousands
of times farther away than the outer planets, an immense jump in scale
that the die-hard geocentrists can be forgiven for finding implausible...
especially since one of their often-unexamined basic assumptions was that
all things in nature had human-related purposes, and it was hard to make
sense of a cluttered solar system surrounded by vast emptiness.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Stars are Suns
Date: Fri, 21 Nov 1997 16:41:37 GMT

In article <6537ik$cp4@marianna.psu.edu>,
Oleg Zabluda  <zabluda@math.psu.edu> wrote:
>Spectroscopy appeared in the mid 1800'. I don't understand why
>not before, since both a telescope and a prism existed for a long
>time before that. Namely since Newton...

Don't forget the problem of optical quality.  When people today see a
prism, it's a crystal-clear block of glass... but early prisms weren't
like that.

The major reason why Joseph Fraunhofer was the first to notice spectral
lines was that he was one of the first optical experimenters with access
to really flawless glass.  Indeed, even he needed some time to convince
himself that the lines were too consistent to be due to subtle flaws in
his prisms.  Any earlier experimenters who noticed the lines undoubtedly
dismissed them as the result of flawed optics.

It was a while after that before flawless optics became sufficiently
widely available for experimenters to notice that the Fraunhofer D line,
a prominent absorption line in the solar spectrum, was at the same
wavelength as a prominent emission line in the spectrum of hot sodium.
(Possibly Fraunhofer himself might have discovered this, had he not died
young.)  This was the first hint that spectral analysis might be possible.

And then it took a bit longer for Kirchhoff to formalize the connection --
that elements absorbed at the same wavelengths they emitted -- and for him
and Bunsen to systematically survey the emission wavelengths of the common
elements and match them up with solar absorption lines.  That was also
roughly the time when large, high-quality lenses were beginning to be
available for telescopes, giving sufficient light-gathering power to make
a bright star's spectrum visible (no photography yet, remember).
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Stars are Suns
Date: Sat, 22 Nov 1997 02:38:24 GMT

In article <bevnsagEK0sJp.MHJ@netcom.com>,
Bev Clark/Steve Gallacci <bevnsag@netcom.com> wrote:
>>Don't forget the problem of optical quality.  When people today see a
>>prism, it's a crystal-clear block of glass... but early prisms weren't
>>like that.
>
>I was under the impression that stellar spectra were done with
>defraction gratings? SA had an article on some 19th C. efforts in the
>making of precision gratings...

Gratings are nearly universal in spectroscopy now, but weren't in the
beginning.  If memory serves -- the handy references don't cover this
topic well -- Fraunhofer also invented the diffraction grating, but it
wasn't until late in the 19th century that Rowland succeeded in making
gratings which were good enough to replace prisms for spectroscopy.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu