Lecture 16 The Measuring the Stars 3/26/2018

Transcription

1 Lecture 16 The Measuring the Stars 3/26/2018

2 Test 2 Results D C B A Questions that I thought were unfair: 13, 18, 25, 76, 77, 80 Curved from 85 to 79

3 Measuring stars How far away are they? How bright are they? How hot? How big are stars? How long do they live? How are they moving? What are their chemical composition? Are they isolated or in clusters? By answering these questions, we not only learn about stars, but about the structure and evolution of galaxies they live in, and the Universe.

4 How far away are stars? Earth-baseline parallax good for solar distances Earth-orbit parallax good for nearby stars

5 Parsecs and Parallax One parsec is defined as the distance required to measure a parallactic angle of 1. Remember 1 (arc second) = 1/60 arc min = 1/3600 degrees 1 distance in pc = parallax (in arc seconds) Closest star, Proxima Centauri, is 1.3 pc away (4.3 ly). Located in the Alpha Centauri complex. Earth-orbit parallax using ground-based optical telescope is good for stars within 30 pc (~100 ly). Milky way ~100,000 ly across

6 Roughly ~100,000 light years

7 Stars within ~13 lightyears

8 Proper motion of a stars Stars have an apparent motion due to parallax, but they also are in motion around larger objects. Radial motion- motion towards or away from the Sun. Measured using Doppler shift. Transverse motion- motion perpendicular to line of sight

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10 Barnard s star After correcting for parallax, Barnard s star still has transverse motion across the sky. It moves 10.3 /yr and has the largest proper motion of any start in our sky. Proper motion is different than total speed in that it does not take into account the radial velocity, but only the transverse velocity.

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12 Apparent Brightness The apparent brightness is defined as how much energy is coming from the star per square meter per second, as measured on Earth. How do we measure this? Remember that a photons energy is related to its wavelength. Count the number of photons/second->get the energy/second for a certain size of detector. If you can measure the distance to the star as well, you can measure the absolute brightness, called luminosity.

13 Two stars can appear to have different apparent brightness even if they are identical. Two stars can have the same apparent brightness but be different stars

14 Luminosity and Apparent Brightness Apparent luminosity is measured using a magnitude scale, which is related to our perception. It is a logarithmic scale; a change of 5 in magnitude corresponds to a change of a factor of 100 in apparent brightness. It is also inverted larger magnitudes are dimmer. Absolute magnitude is defined as the apparent brightness at a distance of 10 pc.

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16 A star s color depends on its surface temperature

17 Stellar Temperatures 2017 Pearson Education, Inc.

18 Categories of Stars: The seven spectral types 2017 Pearson Education, Inc.

19 Stellar Sizes For the vast majority of stars that cannot be imaged directly, size must be calculated knowing the luminosity and temperature:

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21 Binary Stars Most stars are members of a multiple-star system. Meaning their solar system has more than one Sun. Most common is a binary system Visual Binaries: can resolve both stars Spectroscopic binaries: cannot resolve stars but spectrum is periodically Doppler shifted Eclipsing binaries: orbiting edge on in our line of sight

22 Visual Binary

23 Spectroscopic binary

24 Radial Velocity and Mass By measuring the orbital period and semi-major axis the sum of the masses can be measured (Kepler s Laws) D 3 = (M 1 + M 2 )P 2

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27 Hertzsprung-Russell (H-R) diagrams The H-R diagram plots absolute magnitudes against surface temperatures 90% stars fall along main sequence. A star spends 90% of its life on the main sequence. Main sequence stars are burning hydrogen to helium.

28 Giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence

29 H-R Diagram

30 Using the H-R diagram and the inverse square law, the star s luminosity and distance can be found without measuring its stellar parallax 1. Measure the star s apparent magnitude and spectral class. 2. Use spectral class to estimate luminosity. 3. Apply inverse-square law to find distance.