Exploring Stars: Distances and Magnitudes

1. Stars Looking Into the Past Looking Further Into the Past Distance and Speed Birthday star url

2. Star Groups • Constellations – not real groups, they just appear that way. Not bound by gravity • Clusters – are really bound by gravity • Binaries – Most stars occur as two stars circling each other like these two stars Binary star link • Binary motion link The Pegasus Constellation The Pleiades Cluster

3. Stars in different wavelengths • Stars have been studied using telescopes of different wavelengths. • These different “views” of the stars give us information about them • Classzone link different wavelengths

4. Measuring the Stars • Methods to measure stellar distances • Stellar parallax for stars up to 300 light years away • Comparing absolute and apparent magnitudes. This is called spectroscopic parallax • Using periods of variable stars, like Cepheids • Red shifts of very distant objects Stellar distances are measured in parsecs (from parallax seconds) or light years. A light year is about 5.9 trillion miles and 1 parsec = 3.26 light years. Light year link

5. Trigonometric(Stellar) Parallax • Requires very precise measurements of stellar positions, and long baselines • Need telescopes with high resolution, and must observe over several years. • The Hipparchos satellite measured distances to tens of thousands of stars within 1,500 light-years of the Sun.

6. Stellar Parallax • Parallax is the apparent change in position of objects when seen from different positions. DEMO • This change in the position of a nearby star against the background stars allows us to find the distance of the nearby star Parallax Link

7. Using Parallax Below are two star photos taken six months apart and laid atop one another so the background stars (circles) line up. There are two nearby stars also shown . Which of these nearby stars is closer? a) Star b) Star The answer is b since the amount of parallax shift is greater for closer stars. Parallax worksheet

8. Magnitudes • Hipparcos assigned magnitudes to stars during his observations of which stars were first visible after sunset. These were relative brightnesses of the stars. • 1st order stars were brightest, followed by 2nd order, etc. • The difference between a 1st and 2nd order is actually about 2.5 times brighter • Over time, some of these have been changed with better observations • As we have improved technology, we are also able to see dimmer stars • Also, we found brighter stars which appeared dim because they were far away. Some were so bright that they were assigned magnitudes with negative numbers.

9. Apparent Magnitude (m) • The assigned magnitudes are as we see them from earth, so closer stars will obviously appear brighter while farther ones will appear dimmer • Therefore, these are called apparent magnitudes. They change with distance from the star. • Because the brightest are lower numbers, this can be confusing. Remember that the brightest were 1st order and 2nd order were dimmer stars. Look at “magnitude” ditto for help. • So, which is brighter, a +1 star or a +5? • Which is brighter, a +5 or a -5? Yes, +1 Yes, -5

10. Absolute Magnitude (M) • It was found necessary to assign “real” values for magnitudes, independent of distance. • Since magnitude changes with distance, it was decided to assign a reference distance of 10 parsecs, or 32.6 light years and use the magnitude at that distance as a constant. • The absolute magnitude is the brightness of this star if viewed from this reference distance. So, an absolute magnitude of +5 is the brightness of the star at a distance of 10 parsecs. The absolute magnitude doesn’t change since distance is fixed. • The actual energy output from a star is its luminosity. It is related to absolute magnitude. More luminous stars have higher magnitudes.

11. Spectroscopic Parallax • Comparing Apparent and Absolute magnitude( if we can get it) can give us the actual distance to a star. • Examples: 1) If we have a star with an absolute magnitude(M) of 3 and an apparent magnitude(m) of 1, is it closer or farther than 10 parsecs from us? (Think about it. Which is brighter, m or M?) 2) If the star has M = -1 and m = +1, is it farther, closer or = to 10 parsecs in distance? 3) What if M = m? How close is it? It’s closer because it appears brighter than it really is Farther. It doesn’t look as bright as it is. 10 pc

12. Other methods • Cepheid variables and Red shift distance calculations are discussed in other presentations • But, briefly • red shift of a star’s spectrum is also used to study motion of stars towards us or away from us • And Cepheids are stars that vary in brightness (next slide)

13. Cepheid Variable Stars There is a kind of giant star whose surface pulsates in and out with a regular period. That period of pulsation is related to the Luminosity of the star. LMC contains hundreds of known Cepheids all at the same distance. Which allows for robust determination of the Period Luminosity Relationship. How far away are most stars? How likely is it that we will be able to go out and find other life forms? If they are intelligent, how might we find them? How likely are “alien encounters?”

14. Hertzsprung-Russell Diagram • This diagram shows the relationship between the luminosity, magnitude, temperature and spectral class of stars of different masses. Let us discuss these four properties. • Look at the sun. It is a main sequence star, one of many • But it will not stay that way forever HR diagram website

15. Using the H-R diagram • A red giant of spectral type K9 and a red main sequence star of the same spectral type have the same • a) luminosity. • b) temperature. • c) absolute magnitude. • A red giant of spectral type K9 and a red main sequence star of the same spectral type have the same • a) luminosity. • b) temperature. • c) absolute magnitude. WHY?

16. The star ages • As the star ages, it changes to different stages. • These have different properties of size, temperature, color, spectral type and luminosity • HR evolution link

17. Lifetimes of different spectral types.Spectral type, like everything else is determined by the mass of the star. The hottest stars are “O” while the coolest are “M”, as you also see on the H-R diagram. Hotter stars burn themselves out faster.

18. Evolution of stars • This brings us to the big topic of Stellar Evolution – How stars are born, change and eventually die out • Let’s start with their beginnings • Link 1 – for stars like our sun – simple • But not all stars are like the sun. The evolution of a star depends on its mass. • Link 2 • Link 3

19. Stellar evolution again • Classzone stellar evolution link – this link shows examples of stars at different stages in their lives • Their ends depend totally on their mass. • More massive stars are pulled in more by gravity, causing more pressure which increases temperature causing faster reactions

20. Medium and small stars • Medium stars(like the sun) will expand to a red giant - like the red giant here • Then will end up as a planetary nebula with a white dwarf in the center, after the outer layers drift away from the star. • Very small stars never fuse beyond helium, so they go straight from main sequence to white dwarf

21. Stellar fusion • Classzone star activity • Stars fuse hydrogen, then heavier gases, if they are massive enough. link • If they have enough mass, they have a lot of gravity and can pull in their material to cause greater pressure • This increases the temperature which causes increased fusion.

22. Massive stars • These end up as either neutron stars or black holes after a supernova explosion • Links: • Supernovas • neutron star and black holes • black hole (Why does gravity increase?) • Black hole online • Pulsar pursuit PowerPoint

23. Stages of a 25 solar mass (large) star

24. Opposing Forces • Throughout a star’s life there are two forces in opposition – gravity which pulls inward and pressure caused by heat of the star which pushes outward • These two forces work to keep the star in balance. This is called hydrostatic equilibrium • If gravity is greater a star collapses inward • If heat is greater, a star expands outward • Link • This is important. It ties in to feedback, Newton’s Laws, etc.

25. Back to life on Earth • So, what types of stars would be best suited for planets with complex life? • Discussion