Classifying Stars Classifying stars by their spectra was developed by a team of women astronomers (led by Edward Pickering) Wilhelmina Fleming at the Harvard.

Presentation on theme: "Classifying Stars Classifying stars by their spectra was developed by a team of women astronomers (led by Edward Pickering) Wilhelmina Fleming at the Harvard."— Presentation transcript:

1 Classifying Stars Classifying stars by their spectra was developed by a team of women astronomers (led by Edward Pickering) Wilhelmina Fleming at the Harvard College Observatory in the 19th century. Social conventions of the time prevented most women astronomers from using research telescopes or receiving salaries comparable to men’s. Figure Classifying the Spectra of the Stars The modern scheme of classifying stars by their spectra was developed at the Harvard College Observatory in the late nineteenth century. A team of women astronomers led by Edward C. Pickering and Williamina Fleming (standing) analyzed hundreds of thousands of spectra. Social conventions of the time prevented most women astronomers from using research telescopes or receiving salaries comparable to men’s. (Harvard College Observatory)

2 Hot stars give off more light — and most of it at shorter wavelengths — than cooler stars.
Figure 17-7 Temperature and Color These graphs show the intensity of light emitted by three hypothetical stars plotted against wavelength (compare with Figure 5-11). The rainbow band indicates the range of visible wavelengths. The star’s apparent color depends on whether the intensity curve has larger values at the short-wavelength or long-wavelength end of the visible spectrum. The insets show stars of about these surface temperatures. UV stands for ultraviolet, which extends from 10 to 400 nm. See Figure 3-4 for more on wavelengths of the spectrum. (Inset a: Andrea Dupree/Harvard-Smithsonian CFA, Ronald Gilliland/STScI, NASA and ESA; inset b: NSO/AURA/NSF; inset c: Till Credner, Allthesky.com)

3

4 Strength of Absorption Lines
Figure The Strengths of Absorption Lines Each curve in this graph peaks at the stellar surface temperature for which that chemical’s absorption line is strongest. For example, hydrogen (H) absorption lines are strongest in A stars with surface temperatures near 10,000 K. Roman numeral I denotes neutral, un-ionized atoms; II, III, and IV denote atoms that are singly, doubly, or triply ionized (that is, have lost one, two, or three electrons).

5 Infrared Image of Brown Dwarf
T = 5200 K D = 36 LY T = 850 K Figure An Infrared Image of Brown Dwarf HD 3651B The star HD 3561 is of spectral class K, with a surface temperature of about 5200 K. (“HD” refers to the Henry Draper Catalogue.) In 2006 it was discovered that HD 3651 is orbited by a brown dwarf named HD 3651B with a surface temperature between 800 and 900 K and a luminosity just 1/300,000 that of the Sun. The brown dwarf emits most of its light at infrared wavelengths, so an infrared telescope was used to record this image. The hotter and more luminous star HD 3651 is greatly overexposed in this image and appears much larger than its true size. HD 3651 and HD 3651B are both 11 pc (36 ly) from Earth in the constellation Pisces (the Fish); the other stars in this image are much farther away. (M. Mugrauer and R. Neuhäuser, U. of Jena; A. Seifahrt, ESO; and T. Mazeh, Tel Aviv U.)

6 Oh Be A Fine Girl — Kiss Me
Generate Killer Mnemonics Only Brilliant Awesome Females

7 Use Parallax to Measure Distance
R = D/2 Figure 17-1 Parallax Imagine looking at some nearby object (a tree) against a distant background (mountains). When you move from one location to another, the nearby object appears to shift with respect to the distant background scenery. This familiar phenomenon is called parallax. D

8 Stellar Parallax — works for ~ 1000 Stars
Figure 17-2 Stellar Parallax (a) As the Earth orbits the Sun, a nearby star appears to shift its position against the background of distant stars. The parallax (p) of the star is equal to the angular radius of the Earth’s orbit as seen from the star. (b) The closer the star is to us, the greater the parallax angle p. The distance d to the star (in parsecs) is equal to the reciprocal of the parallax angle p (in arcseconds): d = 1/p.

9 Apparent Brightness (Magnitude)
If you know distance and apparent brightness … Then you can calculate ‘absolute brightness' (Luminosity) Figure 17-6 The Apparent Magnitude Scale (a) Astronomers denote the apparent brightness of objects in the sky by their apparent magnitudes. The greater the apparent magnitude, the dimmer the object. (David Malin/Anglo-Australian Observatory)

10 Apparent Magnitudes of Stars in Pleides
Figure 17-6 The Apparent Magnitude Scale (b) This photograph of the Pleiades cluster, located about 120 pc away in the constellation Taurus, shows the apparent magnitudes of some of its stars. Most are too faint to be seen by the naked eye. (David Malin/Anglo-Australian Observatory)

11 Size of a Star Measure Spectrum Measure parallax Measure brightness
Figure Finding Key Properties of a Nearby Star This flowchart shows how astronomers determine the properties of a relatively nearby star (one close enough that its parallax can be measured). The rounded purple boxes show the measurements that must be made of the star, the blue ovals show the key equations that are used (from Sections 17-2, 17-5, and 17-6), and the green rectangles show the inferred properties of the stars. A different procedure is followed for more distant stars (see Section 17-8, especially Figure 17-17). Measure brightness Calculate Radius

12 Hertzsprung — Russell Diagram
Figure 8.7: An H–R diagram is a graph of luminosity versus temperature on which stars are represented as points. Spectral types have been added at the top to show how they are related to temperature. Absolute magnitude has been added at the right to show its relationship to luminosity. Star diameters are not to scale in this schematic diagram.

13 Figure 17-15 Hertzsprung-Russell (H-R) Diagrams
On an H-R diagram, the luminosities (or absolute magnitudes) of stars are plotted against their spectral types (or surface temperatures). (a) The data points are grouped in just a few regions on the graph, showing that luminosity and spectral type are correlated. Most stars lie along the red curve called the main sequence. Giants like Arcturus as well as supergiants like Rigel and Betelgeuse are above the main sequence, and white dwarfs like Sirius B are below it. (b) The blue curves on this H-R diagram enclose the regions of the diagram in which different types of stars are found. The dashed diagonal lines indicate different stellar radii. For a given stellar radius, as the surface temperature increases (that is, moving from right to left in the diagram), the star glows more intensely and the luminosity increases (that is, moving upward in the diagram). Note that the Sun is intermediate in luminosity, surface temperature, and radius.

14 Luminosity Classes — Sun is G2V
Figure Luminosity Classes The H-R diagram is divided into regions corresponding to stars of different luminosity classes. (White dwarfs do not have their own luminosity class.) A star’s spectrum reveals both its spectral type and its luminosity class; from these, the star’s luminosity can be determined.

15 Question What is the spectral type of the Sun? K9WOOF G2V F7V A#1
OB1KNOB

16 Stars Too Far Away to Measure Distance by Parallax
Measure how bright it appears Measure spectrum to get T Use HR diagram to get L, how bright it IS Figure The Method of Spectroscopic Parallax If a star is too far away, its parallax angle is too small to allow a direct determination of its distance. This flowchart shows how astronomers deduce the properties of such a distant star. Note that the H-R diagram plays a central role in determining the star’s luminosity from its spectral type and luminosity class. Just as for nearby stars (see Figure 17-14), the star’s chemical composition is determined from its spectrum, and the star’s radius is calculated from the luminosity and surface temperature. Use Inverse Square Law to get d What about mass? Calculate R

17 Where is the ‘balance point’? Closer to ‘Big’ kid
‘Little’ kid Figure Center of Mass in a Binary Star System (a) A seesaw balances if the fulcrum is at the center of mass of the two children.

18 Binary Stars → Mass Figure 17-19 A Binary Star System
As seen from Earth, the two stars that make up the binary system called 2MASSW J are separated by less than 1/3 arcsecond. The images surrounding the center diagram show the relative positions of the two stars over a four-year period. These images were made by the Hubble Space Telescope (HST), the European Southern Observatory’s Very Large Telescope (VLT), and Keck I and Gemini North in Hawaii (see Figure 6-16). For simplicity, the diagram shows one star as remaining stationary; in reality, both stars move around their common center of mass. (H. Bouy et al., MPE and ESO)

19 Binary Stars Two stars of unequal mass Two stars of equal mass
Star and Large Planet Two equal mass stars ─ elliptical orbits

20 Figure 17-21 The Mass-Luminosity Relation
For main-sequence stars, there is a direct correlation between mass and luminosity—the more massive a star, the more luminous it is. A main-sequence star of mass 10 M (that is, 10 times the Sun’s mass) has roughly 3000 times the Sun’s luminosity (3000 L); one with 0.1 M has a luminosity of only about L.

21 Figure 17-21 The Mass-Luminosity Relation
For main-sequence stars, there is a direct correlation between mass and luminosity—the more massive a star, the more luminous it is. A main-sequence star of mass 10 M (that is, 10 times the Sun’s mass) has roughly 3000 times the Sun’s luminosity (3000 L); one with 0.1 M has a luminosity of only about L.

22 The main sequence is an arrangement of stars according to their
mass. The most massive main-sequence stars have the greatest luminosity, greatest radius, and greatest surface temperature. This is a consequence of the behavior of thermonuclear reactions at the core of a main-sequence star.

23 The Sun’s Lifetime as a Normal Star
How long will the Sun generate energy by fusing hydrogen into helium? The total available energy stored in the Sun in the form of mass, most of which is hydrogen, (according to Prof. Einstein) is— E⨀ = M⨀c2 = 1.99 x 1030 kg x (3 x 108 m/s) 2 = 1.8 x 1047 J The Sun will fuse only 10% of its available hydrogen into helium. This fraction is f =0.10. The fusion process is only 0.7 % efficient, i.e., only 0.7% of the mass of the hydrogen is converted into energy when hydrogen is fused into helium. This conversion efficiency is ε =0.007. The total amount of energy released, ΔE, by nuclear fusion is— ΔE = ε f M⨀c2 = x 0.10 x E⨀ = x 0.1 x 1.8 x 1047 J = 1.25 x 1044 J. The Sun’s luminosity is L⨀ = 3.9 x 1026 J/s. It’s estimated lifetime is—

24 Luminosity LS of a Star of Mass MS
LS = Lʘ (MS/Mʘ)3.8 Figure The Mass-Luminosity Relation For main-sequence stars, there is a direct correlation between mass and luminosity—the more massive a star, the more luminous it is. A main-sequence star of mass 10 M (that is, 10 times the Sun’s mass) has roughly 3000 times the Sun’s luminosity (3000 L); one with 0.1 M has a luminosity of only about L.

25 Lifetime of Mass MS Star
The total amount of energy released by a star is — ΔES = ε f MSc2. Relative to the energy released by the Sun, it is — ΔES / ΔEʘ = MS / Mʘ ΔES = (MS / Mʘ) ΔEʘ The Star’s luminosity is LS, which depends on its mass. Relative to the luminosity of the Sun, it is — LS = Lʘ (MS/Mʘ)3.8 The Star’s lifetime is τS, which depends on its luminosity and mass — τS = ES / LS = τʘ [(MS /Mʘ) / (MS/Mʘ)3.8 ] Thus, the lifetime τS is— τS = τʘ (MS/Mʘ)-2.8 ]

26 Main Sequence Lifetimes
Type Mass (M) Luminosity (L) Temperature (K) Lifetime (Gyr) O 25 80,000 35,000 0.003 B 15 10,000 30,000 0.015 A 3 60 11,000 0.5 F 1.5 5 7,000 G 1 6,000 10 K 0.75 5,000 M 0.03 4,000 200 O, B, A and F0 – F6 stars don’t live long enough for life to develop. But they are a pretty small fraction of all stars.

27 Question A star more massive than our Sun will _.
have a longer lifetime have a shorter lifetime have the same lifetime as the Sun have a longer lifetime or a shorter lifetime depending on its chemical composition live forever

28 Low Mass M-Type Stars? Tidal forces scale as 1/d3. The habitable zone is < 0.5 AU for M-type stars. Thus, tidal forces are 1/ (0.5)3 = 8 x greater than for Earth-like planets 1.0 AU from Sun-like stars. This causes ‘tidal lock’ where one side of the planet faces its parent star!

29 Low Mass K – Type Stars? Planets around cooler, red dwarf stars and brown dwarfs may have much less of the same prebiotic chemicals such as Hydrogen Cyanide that are incorporated into Earth life as brighter, Sol-type stars K and M stars typically emit flares and much uv which would sterilize life. Planets that form around K and M stars tend to be small, lose internal heat too rapidly to remain geologically active.

30 CHZs for Other Stars? - Hart (1979 Icarus, 37, 351-357)
Thickness goes to ZERO for stellar masses less than 0.8 solar masses and greater than 1.2 solar masses! Stellar Mass Class Rin Rout Thickness >1.20 *** Red Giant Too Soon *** 1.20 F 1.15 F 1.10 F 1.05 G 1.00 G 0.95 G 0.90 G 0.85 K K

31 Question Even though K- and M-type stars have very long lifetimes, why might they not have many habitable planets around them? Their habitable zones are too wide Planets don’t form around K- and M-type stars They typically emit flares and an enormous amount of ultraviolet light They contain too few heavy elements to form planets Their habitable zones are very narrow

32 Habitable Planets in Multiple Star Systems?
Estimated fraction of stars in multiple star systems ~ 80% of O, B and A stars. ~ 50% F and G ~ 40% K ~ 25% M Some have planets … but Orbits stable only near one star or far from them all. A multiple star system is as bad for life as its worst star. Multiple stars have more restricted HZ’s and more variable planetary environments. Imagine our solar system with a small star in place of Jupiter! “NASA scientist discovers world with triple sunsets July 13, 2005 The new planet, called HD Ab, is the first known to reside in a classic triple-star system. The masses of the three stars range from two-thirds to about the same mass as our Sun. The planet is slightly more massive than Jupiter ."The sky view from this planet would be spectacular, with an occasional triple sunset," said Dr. Maciej Konacki of the California Institute of Technology, Pasadena, Calif., who found the planet using the Keck I telescope atop Mauna Kea mountain in Hawaii. "Before now, we had no clues about whether planets could form in such gravitationally complex systems."The new planet belongs to a common class of extrasolar planets called "hot Jupiters," which are gas giants that zip closely around their parent stars. In this case, the planet whips every 3.3 days around a star that is circled every 25.7 years by a pirouetting pair of stars locked in a 156-day orbit. Previously, astronomers had identified planets around about 20 binary stars and one set of triple stars. But the stars in those systems had a lot of space between them. “ JPL website

33 Stable Orbits in Binary Systems
If a planet orbits one star in a binary system and the orbital distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. A planet could also orbit both stars if the planet does not come closer to either stars than about 3.5 times the separation distance between the two stars. In star systems with more than two stars, the limits on stable orbital distance are so stringent that the presence of Earth-type planets in a habitable zone are unlikely…but not impossible.

34 The Alpha Centauri System α Centauri A, α Centauri B, Proxima Centauri
dAB = 4.37 LY dC = 4.24 LY G2V Mʘ Lʘ G2V ~1.1 Mʘ ~1.5 Lʘ K1V ~0.9 Mʘ ~0.5 Lʘ M6Ve ~0.1 Mʘ ~10-4 Lʘ Flare star

35 A and B in Elliptical Orbits About CM
The distance between A and B varies between 11 – 35 AU 11 AU 35 AU

36 No ‘Gas Giant’ planets found … what is possible though?
Calculations suggest that stable planetary orbits exist within 2 AU of either Alpha Centauri A or B and beyond 70 AU for planets circling both stars. Under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like our Solar System. No ‘Gas Giant’ planets found … what is possible though?

37 Terrestrial Planet Finder and SIM
A and B were 2 of the top 100 targets selected for NASA's Terrestrial Planet Finder (TPF) to directly image small, terrestrial planets in habitable orbits. A, B and C were "Tier 1" targets selected for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as 3 Earth-masses within 2 AUs of its host star. Both TPF and SIM have been indefinitely postponed due to withdrawal of NASA funding.

38 Pro’s and Con’s Pro’s The AB system is significantly more enriched (1.7 to 1.8 times) in elements heavier than hydrogen ("high metallicity") than our own Solar System. Hence, either stars A or B could have one or two "rocky" planets in orbital zones where liquid water is possible. Age ~ 4850 Gyr Habitable zones: A ~ 1.25 AU B ~ 0.75 AU Habitable zone unaffected by other star. Con’s Orbits might not be stable for more than 250 Myr. Proxima Centauri would have disrupted formation of Oort cloud around AB. Terrestrial planets could be bone dry or lack atmosphere (no cometary bombardment or source of hydrated compounds from beyond ‘ice limit’).

39 α-Centauri A α-Centauri B View from a hypothetical, airless planet orbiting Alpha Centauri A. Alpha Centauri B can be see as a dim red star.

40 But … More Con’s … Only if no further apart than 3 AU
… excludes α Centauri C Essentially independent stars C. Dust disk observed around one star … stars in these systems ~independent. A. Planets could form around both stars but … no habitable zone! B. No dust disk observed … no planets! α Centauri falls in this domain.

41 Question Which type of planetary orbit might possibly be stable within a habitable zone in a wide binary system (a system in which the two component stars are separated by a great distance)? a circular orbit within the habitable zone of one of the stars a large circular orbit around both stars a “figure-eight” orbit around both stars a large elliptical orbit around both stars a decaying orbit that will cause it to fall into one of the stars

42 33 Stars Within 12.5 LY F5, wd Close binary, A entering RG phase
Variable-emits flares SIM ‘Tier 1’ K5, K7 G2, K1 K2 G8 K5

43 Trappist – 1

44 Proxima – b

45 Ross128 – b

46 Top Candidates Top SETI stars (Stars similar to Sun):
Margaret Turnbull of the Carnegie Institute pored over vast amounts of catalogue data and came up with a list of 17,129 stars released in 2003. Top SETI stars (Stars similar to Sun): Beta Canum Venaticorum G0 V, (27 LY), 1.15 Lʘ Gliese 67 G1.5 V MV (41 LY) , 1.45 Lʘ … but has red dwarf companion that orbits 4.3 AU < R < 10.5 AU … but orbital parameters highly uncertain. HD G1-3 V-MV, (44 LY), 1.09 Lʘ … but has red dwarf companion at 41 AU 18 Scorpii G1-5 V-Va, (45.7 LY), 1.06 Lʘ … but might be more variable than Sun and might have stellar companion at 361 AU 51 Pegasus G5 V, (50.9 LY), 1.30 Lʘ … but has 51 Peg b (Bellerophon) 0.5 MJ planet in very close 0.05 AU orbit (1st extrasolar planet found). … precipitated hypotheses of planet migration.

47 Top TPF stars (Stars that could have planets visible to TPF):
Epsilon Indi A K5 V + (T1 V + T6 V), (12 LY), 0.30 Lʘ … 2 brown dwarfs (40-60 MJ) separated by UA. But brown dwarfs could disturb ‘Oort cloud’ and shower Earths with deadly impacts. Maybe worth a look. Epsilon Eridani K2 V, (10.5 LY), 0.30 Lʘ … but low metallicity, high solar activity, variable luminosity, solar wind 30 x greater than Sun’s implies young star. Fast rotator implies planets haven’t formed. Rand Corp says <3% chance of planets! 40 Eridani K1 V + (DA4 + M4.5 eV), (16.5 LY), 0.46 Lʘ , wd and rd flare companions separated by AU. Definitely worth a look! Alpha Centauri B Tau Ceti G8.5 V (11.9LY), 0.52 Lʘ … low metallicity implies no planets but Tau Ceti has more than 10 x the amount of cometary and asteroidal material than does the Sun so if it does … lethal impacts 10 x more frequent!

48 OVERALL PICTURE The evolution of other terrestrial planets will be similar to that of the Earth if inside the CHZ. CHZs are widest around G0 main sequence stars and shrink to zero at F7 at hot end, and K5 at cool end. On the other hand, Hart is likely too conservative … looks like CHZ wider than he thought. Also, the astonishing number of planets found so far gives us hope … but there is another problem … In all cases, the width of CHZ  0.1 AU, suggesting that the average planetary system only had a ~1% chance for an Earth-like planet in the CHZ! "It appears therefore, that there are probably fewer planets in our galaxy suitable for evolution of advanced life than had been previously thought." M. Hart (1979).