Appendices
283
these stars out to ~ 100 parsecs (~300 light-years). How can this range be
further extended?
The first step involves the comparison of a star’s known luminosity with
its observed brightness. The latter is the brightness (or apparent magnitude)
observed at the telescope. It is less than the intrinsic luminosity (or absolute magnitude) of the star because of the inverse-square attenuation with
distance of the observed radiative energy. If we know how bright the star is
intrinsically, we can estimate its distance using the inverse square law.
From measurements on stars in our galaxy within the 300 light-year range
it was discovered that stars of similar particular types have the same absolute
magnitude. These stars are dubbed standard candles. If stars of these types are
observed outside the range where parallax measurements are observable, we
can estimate their distance by assuming their absolute magnitude is the same
as closer stars of the same type, observe their apparent magnitude, and use
the inverse square law to compute their distances. The distance to another
galaxy can be inferred from the distance to particular stars within it.
The term “standard candle” may sound strange in this context. A standard
candle was originally defined as a one-sixth-pound candle of spermaceti wax,
burning at the rate of 120 grains per hour, and was used when comparing the
intensity of light sources. Its output was one candlepower, a unit of luminous
intensity now obsolete, much to the relief of the oceans’ sperm whales. In
astronomy, the term “standard candle” is used to designate a class of objects
whose members have a fixed intrinsic brightness. Two examples are Cepheid
Variable stars and supernovas of a particular type (Type Ia).
Supernova explosions are rare events. The last observation of a type Ia
supernova explosion in our galaxy was made by Johannes Kepler, and others,
on the 9th October, 1604. Earlier, in 1054 AD, Chinese astronomers had
observed another famous supernova explosion, the remnants of which now
make up the Crab Nebula. The extreme brightness of supernovas—whose
peak light output can equal that of the entire galaxy that contains them—
enables us to observe them in distant galaxies, and thereby estimate the
distance to these galaxies.
A.11.1 Measurement of the Curvature
of the Universe
The temperature fluctuations visible in the Cosmic Microwave Background,
and described in Sect. 11.5, can be understood by considering the plasma
that existed before the Recombination Era. The ingredients in this “soup”
283
these stars out to ~ 100 parsecs (~300 light-years). How can this range be
further extended?
The first step involves the comparison of a star’s known luminosity with
its observed brightness. The latter is the brightness (or apparent magnitude)
observed at the telescope. It is less than the intrinsic luminosity (or absolute magnitude) of the star because of the inverse-square attenuation with
distance of the observed radiative energy. If we know how bright the star is
intrinsically, we can estimate its distance using the inverse square law.
From measurements on stars in our galaxy within the 300 light-year range
it was discovered that stars of similar particular types have the same absolute
magnitude. These stars are dubbed standard candles. If stars of these types are
observed outside the range where parallax measurements are observable, we
can estimate their distance by assuming their absolute magnitude is the same
as closer stars of the same type, observe their apparent magnitude, and use
the inverse square law to compute their distances. The distance to another
galaxy can be inferred from the distance to particular stars within it.
The term “standard candle” may sound strange in this context. A standard
candle was originally defined as a one-sixth-pound candle of spermaceti wax,
burning at the rate of 120 grains per hour, and was used when comparing the
intensity of light sources. Its output was one candlepower, a unit of luminous
intensity now obsolete, much to the relief of the oceans’ sperm whales. In
astronomy, the term “standard candle” is used to designate a class of objects
whose members have a fixed intrinsic brightness. Two examples are Cepheid
Variable stars and supernovas of a particular type (Type Ia).
Supernova explosions are rare events. The last observation of a type Ia
supernova explosion in our galaxy was made by Johannes Kepler, and others,
on the 9th October, 1604. Earlier, in 1054 AD, Chinese astronomers had
observed another famous supernova explosion, the remnants of which now
make up the Crab Nebula. The extreme brightness of supernovas—whose
peak light output can equal that of the entire galaxy that contains them—
enables us to observe them in distant galaxies, and thereby estimate the
distance to these galaxies.
A.11.1 Measurement of the Curvature
of the Universe
The temperature fluctuations visible in the Cosmic Microwave Background,
and described in Sect. 11.5, can be understood by considering the plasma
that existed before the Recombination Era. The ingredients in this “soup”