29:50 Modern Astronomy
Fall 1999
Lecture 17 ...October 4, 1999
Observational Indicators of Massive Star Evolution
Watch the skies! Watch the skies!
(1) The bright star in the vicinity of Venus is the star Regulus. October 5 and 6
they will be very close, together with the moon. Regulus is a B7 Main Sequence star
and is 23.8 parsecs from us.
(2) If you have binoculars, now is a good time to observe the planet Uranus at
a distance of 19.2 astronomical units.
For main part of lecture, see notes of October 1, 1999, beginning with section titled ``Observational Verification of Massive Star Evolution''.
One other interesting aspect of pulsar science worth mentioning is the masses
that pulsars can have. Astrophysical theory states that a pulsar exists because there is
a balance between gravity and a force called the neutron degeneracy force.
Neutron degeneracy can hold up a neutron star if it is not too massive, but if
the mass is large enough an equilibrium is not possible, and the star would collapse
to a point.
The result of these calculations are shown in Figure 20-17 of your textbook. It shows
that strangely enough, the more massive a neutron star is, the smaller it is.
Furthermore, for a big enough mass, the radius actually goes to zero!.
Figure 20-17.
What this means is that neutron stars are not possible for masses higher than this
critical mass. If a ``degenerate object'' has more mass than this, it must be
something even more compact than a neutron star. There aren't many possibilities.
The value of this maximum mass is not well known. Theoretical estimates range from a
lower value of 1.5 solar masses to a high of 2.7 solar masses. Observations,
interestingly, heavily favor the lower value. In fact, in the case of the many binary
pulsars for which masses have been measured, all are consistent with a mass of
1.4 solar masses.
Distribution of pulsar masses.