properties of stars
Astronomy Notes
Electromagnetic Spectrum
Order from shortest wavelength/highest energy to longest wavelength/lowest energy:
Gamma rays
X-rays
Ultraviolet
Visible light
Infrared
Microwaves
Radio waves
Key ideas:
Short wavelength → high energy
Long wavelength → low energy
Inverse Square Law
Light becomes weaker as distance increases.
I∝1d2I \propto \frac{1}{d^2}I∝d21
Double distance → 1/4 intensity
Triple distance → 1/9 intensity
Telescope Properties
Objective
Main lens or mirror that collects light.
Larger objective:
Collects more light
Makes dim objects easier to see
Objective area:
A=π4D2A=\frac{\pi}{4}D^2A=4πD2
Light-Gathering Power
Depends on objective diameter.
Larger diameter = more light collected.
Resolving Power
Ability to see fine details clearly.
θR∼λD\theta_R \sim \frac{\lambda}{D}θR∼Dλ
Larger diameter → sharper images
Longer wavelength → lower resolution
Magnification
Makes image larger.
Does NOT increase brightness.
Spectroscopy
Study of light spectra to determine:
Composition
Temperature
Motion
Distance
Types of Spectra
Continuous Spectrum
Full rainbow
Produced by hot dense objects like stars
Emission Spectrum
Bright lines on dark background
Produced by hot thin gas
Electron drops to lower energy level:
Releases light
Absorption Spectrum
Dark lines on rainbow background
Occurs when:
Light passes through cooler gas
Gas absorbs certain wavelengths
Each element has unique spectral lines.
Kirchhoff’s Laws
Hot dense object → continuous spectrum
Hot thin gas → emission spectrum
Continuous light through cool gas → absorption spectrum
Blackbody Radiation
Blackbody:
Perfect absorber and emitter of radiation
Blackbody curves show:
Wavelength
Energy emitted
Hotter stars:
Emit more energy
Peak at shorter wavelengths
Appear blue
Cooler stars:
Peak at longer wavelengths
Appear red/orange
Approximate temperatures:
3000 K → red
4000 K → orange
5000+ K → blue-white
Doppler Effect
Redshift
Object moving away:
Wavelength increases
Light shifts red
Blueshift
Object moving toward:
Wavelength decreases
Light shifts blue
Rotating stars:
One side blueshifted
One side redshifted
Spectral lines smear out
Magnitude System
Apparent Magnitude
Brightness seen from Earth.
Smaller/negative number = brighter
Larger positive number = dimmer
Difference of 5 magnitudes:
100× brightness difference
Brightness ratio:
2.512m2−m12.512^{m_2-m_1}2.512m2−m1
Absolute Magnitude
True brightness of a star.
Defined as:
Brightness if star were 10 parsecs away
Formula:
m−M=5log(d10)m-M=5\log\left(\frac{d}{10}\right)m−M=5log(10d)
mmm = apparent magnitude
MMM = absolute magnitude
ddd = distance in parsecs
Parallax
Method for measuring nearby star distances.
As Earth orbits the Sun:
Nearby stars appear to shift
Closer star:
Larger parallax angle
Farther star:
Smaller parallax angle
Formula:
D∝1pD \propto \frac{1}{p}D∝p1
Arc-Second
Tiny angle measurement.
1 arc-second=13600∘1\text{ arc-second}=\frac{1}{3600}^{\circ}1 arc-second=36001∘
Measuring Distances
Cepheid Variable Stars
Brightness changes periodically.
Period relates to luminosity.
Used for:
Nearby galaxy distances
Supernovae
Exploding stars.
Extremely bright.
Used for:
Very large distances
Hubble’s Redshift
More distant galaxies:
Have larger redshift
Move away faster
Used to measure:
Galaxy velocity
Distance
Characteristics of Stars
Composition
Typical stars:
71% hydrogen
27% helium
Luminosity
Total energy emitted.
Depends on:
Size
Temperature
Size
White dwarfs ≈ Earth size
Neutron stars ≈ 20 km wide
Supergiants much larger than Sun
Temperature and Color
Red stars → cooler
Blue stars → hotter
Approximate:
3000°C → red
20,000°C+ → blue
Important Relationships
Hotter star → bluer
Cooler star → redder
Larger star → often more luminous
Closer star → larger parallax
Moving away → redshift
Moving toward → blueshift