Stars III The Hertzsprung-Russell Diagram

Transcription

1 Stars III The Hertzsprung-Russell Diagram

2 Attendance Quiz Are you here today? (a) yes Here! (b) no (c) here is such a 90 s concept

3 Today s Topics (first half) Spectral sequence and spectral types Spectral classification and Annie Jump Cannon OBAFGKM HR diagram Patterns among the stars Main sequence, giants and dwarfs Variation of properties along the main sequence Stellar clusters Open clusters Globular clusters Cluster lifetimes

4 Spectral Sequence In the late 1800s, Henry Draper and later Edward Pickering and his team of female assistants ( computers ) at Harvard worked classifying thousands of stars by their absorption spectra At first they classified them by the strengths of the hydrogen absorption lines (A, B, C, ) from strong to weak Later, Annie Jump Cannon, who personally classified over 400,000 stars, realized that the sequence could be ordered and simplified as OBAFGKM (Oh, be a fine girl/guy, kiss me) Annie Jump Cannon

5 Spectral Types As temperature increases, atoms in a stellar atmosphere become more and more excited due to more frequent collisions, so the lines in the star s absorption spectrum change Eventually, if the temperature is high enough, some atoms are ionized and their lines disappear By knowing the energies of excitation and ionization of the various elements, it is possible to infer the temperature of the surface of a star Spectral types are subdivided using numbers, e.g., G0-G9, from hotter to cooler within each type This information can also be used to determine the composition of the star >30,000 K 10-30,000 K 7,500-10,000 K 6,000-7,500 K 5,000-6,000 K 3,500-5,000 K <3,500 K

6 Hertzsprung-Russell (HR) Diagram In the early 20th century, two astronomers independently had the idea of plotting stars on a temperature-luminosity plot This diagram is named in their honor a Hertsprung-Russell diagram (HR diagram for short) Note that the x-axis has temperature increasing to the left (backwards) This is because HR actually plotted the stars using a measure of color (spectral type) from O to M (blue to red)

7 Hertzsprung-Russell (HR) Diagram Note that from the Stefan- Boltzmann law (L T 4 R 2 ), we can draw lines of constant radius (Interactive Figure 15.10) As expected, stars in the upper right of the diagram are larger than those in the lower left Thus, those in the upper right are called giants or supergiants Those in the lower left are called dwarfs

8 Stars are not distributed randomly around the diagram >90% of all stars fall along a curved line known as the Main Sequence (Sun, Spica, Vega, Proxima Centauri) These stars are undergoing fusion of H to He in their cores Stars in the upper right are red and blue giants or red and blue supergiants (Rigel, Deneb, Aldebaran, Antares, Betelgeuse) Stars in the lower left are white dwarfs (Sirius B, Procyon B) These categories represent an evolutionary sequence Categorization of Stars

9 Lecture Tutorial: HR Diagram, pp Note: there is another measure of stellar luminosity called Absolute Magnitude. We are not learning about it in this class, and you are not responsible to know about it. For the LT, answer questions about the Absolute Magnitude using the diagrams, but otherwise, don t worry about it. Work with one or more partners - not alone! Get right to work - you have 10 minutes

10 HR Diagram Quiz I What do the colors of stars in the Hertzsprung-Russell diagram tell us? a) The size of the star b) The luminosity of the star c) The surface temperature of the star d) The core temperature of the star e) The mass of the star

11 HR Diagram Quiz II On an HR diagram, stars at the same temperature are found a) aligned horizontally (i.e., next to each other) b) aligned vertically (i.e., one above the other) c) along the main sequence d) There is no relationship between their positions.

12 HR Diagram Quiz III On an HR diagram, stars with the same luminosity are found a) aligned horizontally (i.e., next to each other) b) aligned vertically (i.e., one above the other) c) along the main sequence d) There is no relationship between their positions.

13 HR Diagram Quiz IV 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) Both are the same. d) Neither is the same. e) Not enough information to tell.

14 Properties of Main Sequence Stars As noted previously, there are links between a stars mass, radius, temperature, and luminosity All the stars on the Main Sequence are undergoing fusion of H to He in their cores The sequence goes from Upper left: hot (10-30,000 + K), bright (100-10,000 + L! ), blue, massive ( M! ) stars Lower right: cool (3-4,000 K), dim ( L! ), red, low-mass ( M! ) stars The Sun is somewhere in the middle (5,800 K, 1 L!, 1 M!, yellow)

15 Properties of Main Sequence Stars There are many more cool, red, low-mass stars than hot, bright, high-mass stars There are two reasons for this: 1. It is harder to assemble the material needed for a high-mass star (10-30 M! ) than for a lowmass star (0.1 M! ) 2. High-mass stars live a much shorter time on the Main Sequence (and overall) than lowmass stars

16 Lifetimes of Main Sequence Stars lifetime amount of stuff to create energy rate stuff is used up amount of stuff mass of star M rate stuff used up lum. of star L L M 4 (high - mass) L M 3 (low - mass) so lifetime M M 1 for high - mass stars 4 3 M lifetime M M 1 for low - mass stars 3 2 M

17 Lifetimes of Main Sequence Stars Recall t! 10 billion years lifetime M M 1 for high - mass stars 4 3 M so t(10 M! ) (10 billion) (1/10) 3 (10 billion)/ million years lifetime M M 1 for low - mass stars 3 2 M so t(0.1 M! ) (10 billion) (1/0.1) 2 (10 billion) trillion years 70 (age of the Universe)

18 Main Sequence Quiz I If two stars are on the main sequence, and one is more luminous than the other, we can be sure that the a) more luminous star will have the longer lifetime b) less luminous star is the more massive c) more luminous star is the more massive d) more luminous star will have the redder color

19 Main Sequence Quiz II Stars that begin their lives with the most mass live longer than less massive stars because it takes them a lot longer to use up their hydrogen fuel. a) Yes, with more hydrogen to burn, massive stars can live for billions of years. b) Yes, low mass stars run out of hydrogen very quickly and have very short lifetimes. c) No, stars have similar lifetimes despite their different masses. d) No, more massive stars are much more luminous than low mass stars and use up their hydrogen faster, even though they have more of it.

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21 Homework For homework, complete the ranking tasks, Luminosity, Exercises 2-4, and Stellar Evolution 1 (download from class website)

22 Star Formation

23 Attendance Quiz Are you here today? (a) yes Here! (b) no (c) here is such a 90 s concept

24 Today s Topics II The battle against gravity Interstellar medium Interstellar dust Molecular clouds How stars form from molecular cloud cores The role of gravity Stellar masses Low-mass limit and Brown dwarfs High-mass limit Stellar mass distribution

25 The Battle Against Gravity Recall that a star like the Sun is balancing between the crushing force of gravity, that would like to squeeze it smaller, and the outward pressure of the radiation caused by the nuclear fusion in its core This battle is the life-story of every star, from before its birth to its end as a white dwarf, neutron star, or black hole The balance of a main sequence star is simply an extended interlude in this battle

26 The Interstellar Medium To understand the formation of stars, we have to ask, What are they made of? Answer: About 99% hydrogen and helium gas Where might this gas come from? Answer: The space between the stars is not empty!

27 The Interstellar Medium There are clouds of gas and dust between the stars Like the stars themselves, they are made primarily of hydrogen and helium Since they are colder than stars, we can t see them in visible light The dark patches below are dust particles absorbing the light of background stars

28 The Interstellar Medium Dust particles (carbon and silicate particles the size of cigar smoke or smaller) absorb and scatter light creating dark patches in the sky What do you notice about the image below (besides the hole )? The bright stars are blue = hot = massive = young (~10 6 yrs) The stars near the edge of the dark patch look reddish

29 Interstellar Dust Although interstellar dust particles make up only about 1% of the interstellar medium (which is itself about 1% of the mass of the stars in the Galaxy), they play a disproportionate role in how we study stars and star formation Dust grains vary in size from nm ( µm), similar to the wavelength of visible light ( nm or µm)

30 Interstellar Dust Since the size of a dust grain is approximately the same as the wavelength of light, dust grains are very efficient at absorbing and scattering visible (and even IR) light Furthermore, the degree to which light is scattered is a function of wavelength: bluer light is scattered much more than redder light (Scattering Demo)

31 Interstellar Dust Thus, dust grains redden light from stars and other objects as the light passes through interstellar clouds (explaining the reddened stars at the edge of the cloud) If there is enough dust, everything can be blocked, causing dark patches It is not a coincidence that there are young, blue stars near this dark cloud Stars are forming in these dense clouds of dust and gas (see next slide)

32 Dark Globule Barnard 68 Infrared Visible light

33 Interstellar Dust Emission These same dust particles can emit thermal radiation Typical temperatures are K (~ cooler than a star), so emission peaks longer in wavelength (by Wein s Law), at µm The left hand image is the familiar constellation of Orion (note the nebula) The right hand image is a 100 µm image of the same part of the sky

34 Molecular Clouds Although dust is often easiest to see in the interstellar medium, most (99%) of the material between the stars is gas, primarily hydrogen The densest clouds allow molecules to form, primarily H 2 : molecular clouds Other molecules: CO (shown below), OH, H 2 O, NH 3,, CH 3 OC 2 H 5, HC 11 N The clouds below contain millions of solar masses of material As of May 2016, over 200 interstellar molecules have been detected in space, mostly at radio (rotation) and infrared (vibration) wavelengths

35 Star Formation in Molecular Clouds How do stars form in molecular clouds? As a cloud (or part of a cloud) contracts under gravity, the gas heats up A combination of thermal pressure and pressure from magnetic fields keeps a cloud from collapsing for a time As the cloud continues to contract, it passes a threshold where it keeps collapsing until the densities and temperatures are high enough for nuclear fusion to begin This computer simulation shows how a cloud fragments into small, dense clumps over time. These clumps fragment into dense cores from which stars form

36 Lecture Tutorial: Star Formation and Lifetimes, pp Work with one or more partners - not alone! Get right to work - you have 10 minutes Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you all agree on. Write clear explanations for your answers. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the Lecture Tutorial is asking, ask me for help.

37 Stellar Lifetime Quiz Consider the information given below about the lifetime of three main sequence stars A, B, and C. Star A will be a main sequence star for 2 million years Star B will be a main sequence star for 70 million years Star C will be a main sequence star for 45,000 million years Which of the following is a true statement about these stars? a) Star A has the greatest mass b) Star C has the greatest mass c) Stars A, B and C all have approximately the same mass d) None of the above

38 Stellar Masses If M < 0.08 M! 80 M jupiter, then the temperature will never be high enough to start fusion Such objects, called Brown Dwarfs, are warm from the heat of gravitational contraction but are much fainter than stars

39 Brown Dwarfs

40 Stellar Masses If M < 0.08 M! 80 M jupiter, then the temperature will never be high enough to start fusion Such objects, called Brown Dwarfs, are warm from the heat of gravitational contraction but are much fainter than stars As such cooler objects have been found 2 new spectral classes (L, T) have been added OBAFGKMLT Oh Be A Fine Girl/Guy, Kiss My Lips Tenderly Only Boring Astronomers Find Gratification in Knowing Mnemonics Like This

41 If M > 150 M!, then the large amount of radiation pressure actually blows off some of the material trying to form the star Thus stellar masses are limited to 0.08 M! < M < 150 M! Because of the details of how molecular clouds fragment into smaller clumps and cores, many more low mass stars are formed than high mass stars The same phenomenon occurs throughout nature (e.g., in a rock pile or bag of cookies) The paucity of high mass stars is made more stark by the short lives they lead Stellar Masses