chapter 19.2; surveying the stars & 19.3; variable stars & 19.4 the H-R diagram and cosmic distances

parallax

• the apparent change in direction of a remote object to due a change in vantage point of the observer

• the angle subtended by the baseline

  • as Earth travels from one side of its orbit to the other, it provides us with a baseline of 2AU

3.26 light years

• how far away would a star have to be to have a parallax of 1 arcsecond with a baseline of 1 arcsecond?

• eequal to 3.1 × 10^14 km

• parsec (pc)

distance of a star in parsecs

variable stars

• stars that vary in their brightness

• many of these stars vary on a regular cycle

light curve

• a graph that shows how the brightness of a variable star changes with time

• maximum; is the point of the light curve where the star has its greatest brightness

• minimum; is the point where it is faintest

• period; if the light variations repeat themselves periodically, the interval between the two maxima is the period of a star

cepheid and RR Lyrae

• two special types of variable stars for which measurements of the light curve can give us accurate distances

• pulsating variable stars

  • diameter changes over time, expanding and contracting

cepheid variables

• large, yellow pulsating stars

• names for the first star of the group, Delta Cephei

• several hundred in our Galaxy

• most cepheids have periods in the range of 3-50 days

• luminosities are about 1000-10,000 times greater than that of the Sun

• variations in luminosity range from a few percent to a factor of 10

the period-luminosity relation

• relation allows you to ‘read off’ how bright the star really is

• astronomers can then compare this intrinsic rightness with the apparent brightness of the star

• brighter appearing cepheids always have the longer periods of light variation

RR Lyrae stars

• named for the star RR Lyrae, the best-known member of the group

• more common than cepheids

• less luminous than cepheids

• thousands in our Galaxy

• periods are always less than 1 day

• changes in brightness are typically less than about a factor of two

luminosity classes

• most widely used system of star classification divides stars of a given spectral class into 6 categories

  • Ia: Brightest supergiants

  • Ib: Less luminous supergiants

  • II: Bright giants

  • III: Giants

  • IV: Subgiants (intermediate between giants and main-sequence stars)

  • V: Main-sequence stars