Re: Lubenow

Glenn Morton (
Tue, 24 Dec 1996 13:03:16

Burgy wrote:

>On page 201 he makes the argument (in cnnection with the YEC position) that
>triangulation techniques can only be done to measure out to about 300 light
>years. I seem to remember (perhaps recently) an argument that this technique
>could be used further out than that; don't see how,as the earth's orbit seems
>to be a limiting factor.Does anyone here know about this?Of course,Lubenow is
>not an expert in astronomy, so if he is in error here,it does not invalidate
>the rest of the book particularly.

I cited Lubenow on some list recently. Lubenow ignores such things as the
Supernova 1987a. In 1987 a supernova exploded in the Large Magellanic Cloud.
For the first .6 of a year, all that could be seen was the bright light from
the supernova. But after about 7 months, astronomers began to see a ring of
light around the star which was reflecting the light from the supernova
towards the earth. The travel time from the supernova to the ring allowed
them to determine that the ring was 1.37 light-years in diameter. This is
because the speed of light is constant and the travel time was a little over
.6 of a year.

"The ring, made of material ejected from the supernova's progenitor star
in its red supergiant phase, already girdled the star five thousand years
before it exploded. But it was cold and dark. Only when the radiation from
the supernova blast reached the ring and heated it to 2 x 10^4 K did it become
observable to ultraviolet and visible-light telescopes."~Bertram
Schwarzschild, "Ring Around 1987 Supernova Provides a New Yardstick," Physics
Today Feb. 1991 p. 20

Now the Hubble telescope can measure the angular diameter of the ring.
Schwarzschild writes:

"The Hubble image tells us that the *angular* diameter of the ring is 1.66
arcseconds. It is this combination of an absolute, light-transit-time
measurement with an angular size measurement of the same celestial object that
yields an intergalactic distance determination of unique precision." Ibid.

This is pure trigonometry like that practiced by the Egyptians long ago. The
distance is 169,000 light-years away.

It looks like (drawing courtesy of K.Smith on Talk Origins):

S = supernova, R = ring, E =earth.

S-------------R What was observed was the
| / angle SER, and the time light
| / took to travel from S to R
| /
| / From the angle we can get
| / the ratios of the travel
| / times from S to E and from
| / S to R. Since the second of
| / these is known, we can
| / calculate the first.
| /
| /
| /
| /

There is also another way to get this type of information from supernova which
do not have gas around them. The flash of the supernova explosion speeds out
at light-speed. Some of it reflects off of interstellar dust. This can be
observed over a period of time. They call this a light-echo. The geometry is
identical to the above drawing except that the ring continues to move outward.

At any given time, we can measure the time from the supernova and the angular
diameter of the echo and can then know how far away the supernova is. In the
case of 1987a the distance calculated is consistent with the ring value.

A final method involves radiointerferometry. If you know how rapidly an
expanding ball of gas (ejecta) from a supernova is moving (and we can know
this by measuring the red shift of the gas) we can then know how big the ball
is over time. By using radiointerferometry, astronomers have been able to
measure the angular diameter of the ejecta over time. This allows direct
triagulation to calculate the distance of the object. In the case of
supernova 1993j it calculates around 12-13 million light-years distance.
Bartel et al write:

"We combine the angular expansion rate determined by the VLBI
data with the optically derived expansion speed to estimate a
distance to M81 of 4.0 +/- 0.6 Mpc, consistent with the value
obtained from measurements of Cepheid variables in M81, 3.63 +/-
0.34 MPC."~N. Bartel et al, "The Shape, Expansion Rate and
Distance of Supernova, 1993J from VLBI Measurements," Nature,
368, April 14,1994, p. 610

An earlier report of the same object:

Distance to M81 via triangulation of expanding shell of SN1993J
at 8 months old, the shell moving at 16,000 km/s ring had a
diameter of 478+/- 8 microarc sec. Sept. 26, 1993.
on Nov. 22, 1993, ring had diameter of 581+/- 35 microarc seconds
J. M. Marcaide et al, "Discovery of shell-like radio-structure in
SN1993J", Nature, 373, Jan. 5, 1995, p.44-45. This calculates a
distance of 3.8+/- .8 Mega parsecs to the supernova.

Thus the claim that parallax can not be used for object greater than 300 light
years is wrong.

Merry Christmas


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