2 Chapter 2 – How do you measure stars?
OpenStax Astronomy Chapter 19: Celestial Distances
OpenStax Astronomy Chapter 17: The Brightness of Stars
OpenStax Astronomy Chapter 5: Radiation and Spectra
Distance to Stars – Parallax
I. the to Stars
A. : brains from the and eyes to estimate nearby
1. Only works for distances (about 10 yards), because limited by distance between your eyes
B. Astronomers from Earth apart to find to stars
1. As orbits the Sun, its changes
2. like nearby stars their positions against stars
3. gives the biggest in viewpoint and most accurate distance
II.
A. in position of an object by a change in the of the
B. Only way to measure to a nearby star
1. There are other indirect ways to measure distance to stars that are away
C. : angle made between Sun, and
D. Parallax Angle is proportional with
1. Angle gets as distance gets
E. The angles of real stars are
1. Use instead of degrees to measure the angle
2. = 1/60 of a degree and = 1/60 of an arcminute or 1/3600 of a degree
F. : distance at which angle equals
1. parsec (pc) = 19 miles or 3.3 light-years (ly)
G. If parallax angle is arcsec, then distance than 1 pc
1. Because parallax angle means distance
H. Can parallax for stars to a few parsecs (1000 ly)
Which of the following stars is closest to us?
A. Procyon (parallax angle = 0.29”)
B. Ross 780 (parallax = 0.21”)
C. Regulus (parallax = 0.04”)
D. Sirius (parallax = 0.38”)
Brightness of Stars – Magnitudes
III. of a Star
A. depends on your from an object
1. away, looks
2. Example:
B. is measured in
1. objects have a magnitude
2. (logarithmic) scale: magnitude of is about times than magnitude of
3. of magnitude for stars
C. : how the star in the sky
1. number is and can be
a. Example: Full moon has a magnitude of
2. Hipparchus classified brightest stars he could see as magnitude and faintest as 6th magnitude
3. On modern scale, brightest star (Sirius) actually
D. : how bright the star at a of parsecs from us
1. Puts stars in so can brightness
2. This gives us an objective scale or ruler
Vega has an apparent magnitude of 0.03. If it were physically moved twice as far from Earth as it is now, which of the following would occur?
A. Apparent magnitude number would increase
B. Apparent magnitude number would decrease
C. Apparent magnitude number would stay the same
If a star has an apparent magnitude of 2.0 and an absolute magnitude of 2.0, what is the distance to the star?
A. 1 parsec
B. 2 parsecs
C. 10 parsecs
D. 20 parsecs
E. Need more information to figure out distance.
Brightness of Stars – Luminosity
IV.
A. A star’s brightness
1. The at which a star
B. (Magnitude) depends on both and
1. A star could have a luminosity be away
C. Measure in units of (Lsun)
1. The luminosity =
2. 1 Lsun = 4 x 1026 Watts = 3000 trillion light bulbs
3. The most luminous stars are (1,000,000) Lsun and the least luminous are Lsun
4. More stars than high-luminosity
Colors of Stars
How do we measure luminosity?
V.
A. Measuring the of a star tells us the temperature
1. Then, can also figure out for most stars (HR Diagram)
B. : “Hotter means ”
1. Blue is also usually luminous
2. Blue is than which is hotter than
C. Stars emit light too
1. stars give off more light, and are more luminous overall
2. stars emit more light
3. stars are than the Sun, very hot are rare
How do we measure color?
VI. and Light
A. are charged particles around a nucleus of an atom
1. Electrons surround the nucleus in a , not distinct orbits
B. Electrons can only have certain amounts of
1. energies are allowed
2. Analogy:
C. Amounts of energy, or , are for every
D. Atoms can or light when electrons energy levels
Which of the following is a true statement about electrons?
A. Electrons are positively charged particles in the nucleus of an atom.
B. Electrons move freely inside the nucleus of an atom.
C. Electrons have specific amounts of energy unique to a particular type of atom.
D. Electrons can only absorb light, not emit it.
VII. and Lines
A. : electron to a energy level
1. Lost energy as a packet of light called a
2. Photon’s determines its and
a. energy is
b. atoms emit different of light
3. Happens all the time, spontaneously, no trigger
4. Example:
B. (Excitation): electron goes to a energy level
1. Photon to electron
a. disappears and gains energy at same time
2. Photon’s energy must energy between the two
a. Photons with energies are
When an electron drops to a lower energy level, what happens?
A. A proton is emitted from the nucleus
B. The electron disappears
C. A photon is absorbed
D. A photon is emitted
VIII. Types of
A. : light from an object over wavelengths
1. Like a from a prism
B. : on dark background, from gas emitting specific photons
1. Color/wavelength of light emitted determined by photon’s (difference between energy levels)
C. : (missing colors) on continuous spectrum from gas absorbing photons with a energy/color/wavelength
D. lines for each type of atom (element)
IX. Spectra
A. produce light/energy at by fusion in their cores
B. light by atoms in outer layers or of star producing an
C. Use absorption lines to determine a star’s
Which type of spectrum would you see if you look at the light from a hot low density gas such as an interstellar cloud?
A. Dark line absorption spectrum
B. Bright line emission spectrum
C. Continuous spectrum
Tutorial Activity – Parallax
1. Draw a straight line connecting the Earth and the star to make a triangle with the Sun, Earth and star on the figure below. The parallax angle is the angle closest to the star. Label it on your diagram.
2. If the distance to the star is 1 parsec, how big is the parallax angle?
3. Draw another star on the diagram that is farther away (by an inch or two) than the star shown. Draw a straight line connecting Earth and the star just as you did before. Label the parallax angle for the second star.
4. Is the parallax angle for the farther star bigger or smaller than the first one?
5. How does the distance of the second star compare to the distance of the first one?
Tutorial Activity – Spectra
“Elementary Astronomy Worksheet Handout 10: Light, Spectra” (modified by Kaisa E. Young) by Catherine Whiting via OER Commons, licensed under CC BY 4.0 https://oercommons.org/courses/elementary-astronomy-worksheets
1. What causes an atom to emit light?
2. Make a sketch or clearly describe each type of spectrum and give an example of an object or situation that would emit such a spectrum.
• Continuous
• Emission Line
• Absorption Line
3. Why does an absorption spectrum reveal the chemical composition of an object?
4. What kind of spectrum do we see from the Sun and other stars? Why?
