PG. 127

Measuring the Stars

Groups of stars

Long ago, people grouped bright stars and named them after animals, mythological characters or every day objects.

Called constellationsThere are 88 constellationsConstellations that appear to move around the north pole

are called circumpolar (Big Dipper)Classified as summer, fall, winter, and spring constellations

Star clusters

Groups of stars that are gravitationally bound to one another are called clusters

Open cluster- stars that are not densely packed

Globular cluster-stars that are densely packed into a spherical shape

Binaries

Binaries

Two stars that are gravitationally bound that orbit a common center of mass are binaries

More than half the stars are binaryAppear to be single stars

Stellar positions and distances

Two units to measure long distances Light Year (ly) =9.461 x 1012 km) Parsec (pc)= 3.26 ly

Scientists use parallax to calculate the distance to a star

The closer the star, the larger the shift

Properties of Stars

Diameter, mass, brightness, energy output, surface temperature, and composition.

Can be smaller than the Sun or bigger (our Sun is average)

Brightness is measured using magnitudeApparent magnitude relies on what we seeAbsolute magnitude takes into account how far a

star is from Earth (can only be calculated if you know the distance of the star)

Luminosity measures the energy output and you must know the apparent magnitude

Spectra of stars

The rainbow you see when you shine white light through a prism is called a spectrum.

3 types: continuous, emission, and absorptionStars have an absorption spectrum

Classification of stars

Stars are assigned spectral types (O, B, A, F, G, K, and M)

Each spectral type is then subdivided into more specific divisions using numbers 1-9

O stars are the hottest and M the coolestHotter stars have simple spectra while cooler

stars have more lines in their spectraOur Sun- G273% Hydrogen and 25% helium and 2% other

elements

Wavelength shifts

Shifts in spectral lines happen when there is motion between the source of light and the observer

If the star is moving toward the observer, the spectral lines are shifted toward shorter wavelengths (called blueshifted)

Star is moving away, wavelengths become longer (redshifted)

H-R Diagrams

Properties of mass, luminosity, temperature, and diameter can be related and shown on a graph called the Hertzsprung-Russell diagram (H-R diagram)

Absolute magnitude is plotted on the vertical axis and temperature or spectral type is plotted on the horizontal axis

90 % of stars fall on a strip of the H-R diagram called the main sequence (runs from upper left corner to lower right)

Upper left = hot, luminous starsLower right= cool, dim stars

Classification on the H-R

Upper-right= cool but luminous; very large; red giants (100x larger than the Sun)

Lower-left= hot but dim; very small; white dwarfs

Use the spectra to answer the question.

Each element absorbs light at a particular wavelength. This absorption of light produces dark lines in spectra. Scientists analyze spectra to identify the composition of a star. According to the spectra shown, which elements does the unknown star contain?

1.A. calcium and sodium2.B. calcium and iron3.C. iron and magnesium4.D. sodium, iron, and magnesium