Astronomers Re-measure the Universe with Hubble Space Telescope (Forwarded)

McDonald Observatory
University of Texas

Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

AUSTIN, Texas -- University of Texas at Austin astronomers are using Hubble
Space Telescope (HST) to improve measurements of vast distances in space, which
could greatly increase the accuracy of knowledge in all areas of astronomy from
understanding how stars evolve to the size and age of the universe itself.

Fritz Benedict, Barbara McArthur, Tom Barnes and colleagues are shoring up the
wobbly "extra-galactic distance ladder" by measuring the tiny apparent motions,
or "parallax," of a particular kind of star called "Cepheid variables."

"HST is the only telescope on Earth or in space that can do this with the
required precision right now," Benedict said. "Obtaining these parallaxes is
extremely difficult, equivalent to measuring the size of a quarter seen from
3,000 miles away."

The project, which ranked first among 1,100 proposals by astronomers for use of
HST this year, continues later this month with more Hubble observations.

Cepheid variable stars are one tool that astronomers use to measure vast
astronomical distances. They work well for this because the rapidity with which
their light-output varies tells scientists their intrinsic brightness. This
interdependence is called the "period-luminosity (PL) relationship." So
astronomers can measure the period of variation for a Cepheid variable star in a
galaxy and deduce that galaxy's distance from knowledge of the luminosity of a
Cepheid with that period.

Cepheids make up one rung on the extra-galactic distance ladder that astronomers
use to measure distance to objects outside our own Milky Way galaxy. In this
ladder, each rung is a type of distance measurement that is the basis for the
next rung above it, to measure out to farther distances.

One of the lower rungs is knowledge of the distance to the Large Magellenic
Cloud (LMC) -- one of the satellite galaxies of the Milky Way. Astronomers'
knowledge of the LMC's distance is based in large measure on Cepheids inside
that galaxy. The problem is, those Cepheids are not made up of the same stuff as
the ones in our galaxy. So astronomers aren't sure if the P-L relationship
really works right on them.

The team is working to eliminate the LMC rung from the distance ladder and to
replace it with something sturdier. They're using HST to directly measure the
distance to 10 Cepheid variable stars inside our own Milky Way galaxy.

"By doing this we can compare the direct distance measurement with the one
predicted by astronomers' best calculation of the Cepheid P-L relationship --
revealing any discrepancies and allowing for necessary adjustments in that
calculation," said Barnes. McArthur added, "Cepheids will then become a better
yardstick."

For this study, the team is using HST to make extremely precise measurements of
the location of each of the 10 Cepheids at various times over two years. In
comparing earlier observations to those taken later, each star appears to have
moved. This apparent motion is called "parallax."

"Trigonometric parallax -- watching a star seeming to move from side to side
because the Earth orbits around the Sun -- is the only fundamental method of
getting Cepheid distances and luminosities free from complicating assumptions,"
Benedict said.

His team is making the measurements using HST's Fine Guidance Sensors (FGS) --
the instruments whose primary reason for being is to enable HST's cameras and
spectrographs to lock onto their targets. However, this "bonus science" with FGS
was planned from the start. Benedict helped ensure that the FGS could be used
for parallax work, and has helped in planning their use for more than two decades.

"Because of the great demand for HST time, we can do these measurements only for
a small number of stars in the Milky Way, fewer than a dozen," Benedict said.
"In the future, SIM can do this for a lot more stars."

SIM, the Space Interferometry Mission, is a future NASA space observatory, but
one whose final results will not be available until 2015.

With this HST project, Bendict says, "I'm happy that we'll have good results in
two years instead of 12."

In message , Andrew Yee
writes
McDonald Observatory
University of Texas

Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

One of the lower rungs is knowledge of the distance to the Large
Magellenic Cloud (LMC) -- one of the satellite galaxies of the Milky
Way. Astronomers' knowledge of the LMC's distance is based in large
measure on Cepheids inside that galaxy. The problem is, those Cepheids
are not made up of the same stuff as the ones in our galaxy. So
astronomers aren't sure if the P-L relationship really works right on them.

If Cepheids in the LMC aren't the same as the ones in our galaxy, is
there any reason to think that Cepheids in galaxies in the Virgo cluster
are ???
--
Rabbit arithmetic - 1 plus 1 equals 10
Remove spam and invalid from address to reply.

Jonathan Silverlight wrote:
In message , Andrew Yee
writes
McDonald Observatory
University of Texas

Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

One of the lower rungs is knowledge of the distance to the Large
Magellenic Cloud (LMC) -- one of the satellite galaxies of the Milky
Way. Astronomers' knowledge of the LMC's distance is based in large
measure on Cepheids inside that galaxy. The problem is, those Cepheids
are not made up of the same stuff as the ones in our galaxy. So
astronomers aren't sure if the P-L relationship really works right on them.

If Cepheids in the LMC aren't the same as the ones in our galaxy, is
there any reason to think that Cepheids in galaxies in the Virgo cluster
are ???

The concern is that Cepheid properties might vary somewhat with stellar
metal abundance, wtih the LMC being down by ~3x with respect to our
region of the Milky Way disk. In this property, various Cepheids in
Virgo would lie mostly between solar and LMC abundances, since many are
in the outer disks of luminous spirals. Cepheids are massive, hence
short-lived, and their chemistry will reflect the current state of the
interstellar medium. Since many spirals have gradients in ISM metal
abundance (decreasing outward), the HST data themselves have been able
to put limits on such changes, since it sees the same period-luminosity
relation for radial subsets of Cepheids in several galaxies.

Still, it's always really really good to check important conclusions
in as many ways as one can.

Bill Keel

Jonathan Silverlight wrote in message ...
In message , Andrew Yee
writes
McDonald Observatory
University of Texas

Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

One of the lower rungs is knowledge of the distance to the Large
Magellenic Cloud (LMC) -- one of the satellite galaxies of the Milky
Way. Astronomers' knowledge of the LMC's distance is based in large
measure on Cepheids inside that galaxy. The problem is, those Cepheids
are not made up of the same stuff as the ones in our galaxy. So
astronomers aren't sure if the P-L relationship really works right on them.

If Cepheids in the LMC aren't the same as the ones in our galaxy, is
there any reason to think that Cepheids in galaxies in the Virgo cluster
are ???

Don't worry dear Jonathan, You always can rely on my theoretical
Hd=4.111 billion years Hubble wavelength time constant and you
can calculate the distances from the observed z, using
D=4,111,000,000*ln(z+1)/ln(2) [light years] (making some corrections
for relative motions...)!

Cheers!
Aladar
http://www.stolmarphysics.com

On Wed, 10 Dec 2003 12:10:53 -0500, Andrew Yee
wrote:

"HST is the only telescope on Earth or in space that can do this with the
required precision right now," Benedict said.

I'm curious - what prevents a ground-based telescope with adaptive
optics from being used for this? I remember reading that, for example,
the Keck telescopes can get diffraction-limited images with adaptive
optics, which should make their resolution four times as good as
Hubble's; or is it more complicated than that?

--
"Sore wa himitsu desu."
To reply by email, remove
the small snack from address.
http://www.esatclear.ie/~rwallace

Russell Wallace wrote:
On Wed, 10 Dec 2003 12:10:53 -0500, Andrew Yee
wrote:

"HST is the only telescope on Earth or in space that can do this with the
required precision right now," Benedict said.

I'm curious - what prevents a ground-based telescope with adaptive
optics from being used for this? I remember reading that, for example,
the Keck telescopes can get diffraction-limited images with adaptive
optics, which should make their resolution four times as good as
Hubble's; or is it more complicated than that?

Small-field astrometry has been exploited magnificently in the
Galactic Center field. Other applications run into lack of suitable
reference stars within the field of good image correction. What's
worse, the point-spread function from AO doesn't just get bigger
as you look farther away from the reference object, the first-order
effect is that it becomes asymmetric (mostly elongated radial to
this distance). This comes about because a single adaptive element
can correct for phase scrambling at one atmospheric level, getting
progressively worse for distortions produced at some other level.
The buzzword for going beyond this is "multiconjugate adaptive optics",
which may require something like a hexagon of laser guide stars
around the direction of interest. There is a Canadian group, in
particular, trying to get this working (on a decade timescale?)
for instruments on Gemini.

Bill Keel

"William C. Keel" wrote in message ...
[...]

The concern is that Cepheid properties might vary somewhat

[...]


Still, it's always really really good to check important conclusions
in as many ways as one can.

Bill Keel

I wonder how could Hubble come-up with a 500 km/s per Mpc value from
the same thing, which is now used to support a 72(!) km/s per Mpc?

Do you mean "really really good to check important conclusions", like
big bang and expanding Universe - concluded solely on the basis of
Hubble redshift? Like, look for other possible causes of Hubble
redshift? Like - bingo(!) - photon energy loss during progression?!

Or may be even think a little before jumping to "non-barionic dark
matter" and "dark energy" non-sensical -- idiocracy?! [Not to mention
the quark and gluon fun...]

Ah, you can't say that...

Cheers!
Aladar
http://www.stolmarphysics.com

In message , Aladar
writes
"William C. Keel" wrote in message
...
[...]

The concern is that Cepheid properties might vary somewhat

[...]


Still, it's always really really good to check important conclusions
in as many ways as one can.

Bill Keel

I wonder how could Hubble come-up with a 500 km/s per Mpc value from
the same thing, which is now used to support a 72(!) km/s per Mpc?



According to Gale Christianson's "Edwin Hubble: Mariner of the Nebulae"
he later revised it _upwards_ to 558 km/s/Mpc.
He was only using a very limited data set, of relatively bright and
close galaxies (46 in 1929). The fact that there were two different
types of Cepheids was announced over 20 years later, and doubled the
distance scale.
I'm sure his measurements are available somewhere.
I haven't (yet ?) been able to confirm this, but I wonder if the reason
that he considered a cause for the red shift other than recession was
that he was aware that the figure he found meant that the universe was
less than 2 billion years old, which was known to be absurd. A figure of
50 to 100 doesn't have this problem.
--
Rabbit arithmetic - 1 plus 1 equals 10
Remove spam and invalid from address to reply.

Jonathan Silverlight wrote in message ...
In message , Aladar
writes
"William C. Keel" wrote in message
...
[...]

The concern is that Cepheid properties might vary somewhat

[...]


Still, it's always really really good to check important conclusions
in as many ways as one can.

Bill Keel

I wonder how could Hubble come-up with a 500 km/s per Mpc value from
the same thing, which is now used to support a 72(!) km/s per Mpc?



According to Gale Christianson's "Edwin Hubble: Mariner of the Nebulae"
he later revised it _upwards_ to 558 km/s/Mpc.
He was only using a very limited data set, of relatively bright and
close galaxies (46 in 1929). The fact that there were two different
types of Cepheids was announced over 20 years later, and doubled the
distance scale.
I'm sure his measurements are available somewhere.
I haven't (yet ?) been able to confirm this, but I wonder if the reason
that he considered a cause for the red shift other than recession was
that he was aware that the figure he found meant that the universe was
less than 2 billion years old, which was known to be absurd. A figure of
50 to 100 doesn't have this problem.

Interesting! I wonder... may be --- as a very distant possibility ...

just to play with the idea...

hm, how to bring it to you ...
without major pain???

this 72 km/s per Mpc is just out of the blue? To avoid the slight
problem that we see older galaxies - even if fudging the Hubble law
with the assumption of 'evolution' - than we calculate the 'age' of the
Universe?!

Now, on the other hand, there is a perfectly good, coherent
representation of matter structure [a candy for the correct answer!]
which results in the photon energy loss with an exponential to
the distance rate - z =2^(t/Hd)-1 where t is the time of photon
travel and Hd =4.111 bly Hubble photon wavelength doubling time
constant. It is around 170 km/s per Mpc for the linear approximation
for very small redshifts.

But you don't want to look at it, because it burries the bigbangology!

Cheers!
Aladar
http://www.stolmarphysics.com

In message , Aladar
writes

Now, on the other hand, there is a perfectly good, coherent
representation of matter structure [a candy for the correct answer!]
which results in the photon energy loss with an exponential to
the distance rate - z =2^(t/Hd)-1 where t is the time of photon
travel and Hd =4.111 bly Hubble photon wavelength doubling time
constant. It is around 170 km/s per Mpc for the linear approximation
for very small redshifts.

I thought your figure was Hd = 4.234 billion
years. Given that you're quoting it to 4 decimal places, isn't that a
rather large difference?
--
Rabbit arithmetic - 1 plus 1 equals 10
Remove spam and invalid from address to reply.