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THE SUN AND ITS RELATION TO OTHER STARS 1. Stellar astronomy Well discuss properties of solar-type and other stars – lets first review some preliminaries:

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Presentation on theme: "THE SUN AND ITS RELATION TO OTHER STARS 1. Stellar astronomy Well discuss properties of solar-type and other stars – lets first review some preliminaries:"— Presentation transcript:


2 Stellar astronomy Well discuss properties of solar-type and other stars – lets first review some preliminaries: Naming of stars Stellar distance scale: parsecs Properties: Stellar spectral classes Stellar luminosity classes Stellar magnitudes Stellar colours and relation to T eff HR diagram 2

3 Naming of stars Stars assigned letters (Greek or Roman) or numbers and sometimes names (Arabic, Latin, or Greek) The names of most (apparently) brightest stars are a letter/number followed by the genitive of the Latin name of a constellation, boundaries of which are defined by the IAU, e.g. Betelgeuse = α Orionis, abbreviated α Ori. Generally brightest 24 stars have Greek letters α, β, γ, δ, ε... ω Variable stars given Roman letters (R, S, T,... Z, then RR, RS, RT,... RZ, SS, ST,... SZ,.... ZZ) 3

4 Stellar distance scales Stellar distances are reckoned in parsecs: 1 pc = the distance at which a stars annual parallax π = 1 arcsecond, i.e. at the star, the radius of the Earths orbit (150 x 10 6 km) subtends an angle of 1 arcsecond. 4 2π2π Note: 1 parsec = 3.26 light year = 3.1 x km

5 Stellar spectral classes: notation Visible-light spectra of stars can be arranged in a sequence: O-B-A-F-G-K-M. This is closely related to the effective temperature: Spectral class T eff (K) O40,000 B28,000 A10,000 F 7,400 G 6,000 K 4,900 M 3,500 Note: spectral types are subdivided (e.g. B0, B1, B2...B9): spectral classes in this table are B0, A0, etc. 5

6 Stellar Luminosity classes Stellar spectra also give an indication of whether stars are on the main sequence or are evolved, i.e. have reached the end of their main sequence lifetime and have evolved to become giants or supergiants. Notation for luminosity is: class I (supergiants), II (bright giants), III (giants), IV (subgiants) and V (main sequence/dwarf stars). The Sun is a G2V star – sometimes called a dG2 star. Its T eff is 5778K. 6

7 Stellar magnitudes In visible-light range, a stars apparent brightness defined by the flux in the spectral region nm on a magnitude (V) or logarithmic scale. We define the V magnitude of the star Vega to be V = 0. The V magnitude of any other star is V = 2.5 [log I(Vega) – log I(star)] where I = energy flux in the nm range in SI units: W m -2. So YZ Canis Minoris (YZ CMi) has V = 11.2: therefore: I (YZ CMi)/ I (Vega) = /2.5 = 3.3 × Note: 5 magnitudes = factor of 100 in brightness. Magnitudes are on a decreasing flux scale: the brighter the star, the smaller is its apparent magnitude. Some stars have negative magnitudes (e.g. Sirius, V = ). 7

8 Absolute magnitudes A stars absolute magnitude is the apparent magnitude it has at a distance of 10 pc. Suns absolute magnitude M V = (its apparent magnitude from Earth is m V = -26.7). For comparison: Sirius has M v = (brightest star in sky) α Cen M V = (Suns twin star) Proxima Cen M V = 15.5 (nearest star) Barnards star M V = 13.5 (largest proper motion) 8

9 Other visible-light magnitudes B (blue) magnitude is measured by flux in the nm range. U (ultraviolet) magnitude is measured by flux in the nm range. Note: U magnitude measures near ultraviolet radiation: wavelengths much greater than UV wavelengths measured by spacecraft. Human eye sensitivity (dark-adapted) is approximately nm. 9

10 Stellar colours U, B, V magnitudes of a star are a measure of its brightness in three different wavelength bands. So U – B and B – V are measures of the stars colour. In particular B – V is often used as a proxy (substitute) for spectral type or effective temperature. Thus for YZ CMi, spectral type is dM4, B-V = (i.e. V is brighter than B), and T eff = 2900K. 10

11 Hertzsprung-Russell Diagram HR diagram for nearby stars 11 4M ʘ 1M ʘ 0.25 M ʘ 0.5 M ʘ Stellar masses indicated in units of 1 solar mass = 1 M ʘ

12 Review of Solar Properties Compared with most stars in the Milky Way galaxy, Sun has medium mass (1 M ʘ = 2 × kg). Sun is mid-way through its life-cycle: age = 4.6 Gyr (expected total lifetime before red giant phase =10 Gyr). Suns interior made up of radiative zone (inner 0.7 R ʘ ) and convective zone (outer part, 0.7 – 1.0 R ʘ ). Solar activity: hot corona (temperature rises with height over chromosphere/transition region to corona); active regions (complex magnetic field); flares; coronal mass ejections. So although photosphere is not very hot (6400 K), its atmosphere (upper chromosphere/corona) is hot and an emitter of ultraviolet, X-ray and radio emission. 12

13 Evidence for solar activity in visible spectrum The Suns visible spectrum has very little evidence of a high-temperature chromosphere, even less of a very hot corona. The only (very subtle) evidence is that strong absorption spectral lines of ionized Ca (Ca + ) – the Ca II H and K lines – have profiles showing small emission peaks, especially over active regions. These peaks arise in the chromosphere. Wavelengths: Ca II K line nm, Ca II H line nm. 13

14 Solar-type stars Many stars with cooler effective temperatures than Suns show the Ca II H and K lines as strong emission lines, not absorption lines at all. Even the Hα and other Balmer lines of H are commonly in emission for such stars. These stars are normally very dim, cool dwarf stars at the bottom of the H-R diagram – they are called dM stars. If a dM star has the Hα line in emission, then it is called a dMe star. There are correspondingly dK and dKe stars 14

15 Properties of dMe stars Photospheric temperatures may be so low (<3000 K) that molecules form – e.g. TiO (titanium oxide), ZrO (zirconium oxide) plus more usual molecules like H 2. (Molecules do exist in the Sun, near the temperature minimum: e.g. H 2 and CH.) Visible-light spectra show emission mostly in red part of spectrum, with strong TiO or ZrO molecular bands with strong emission lines – Ca II H and K, Hα and other Balmer lines. Very often dMe stars are in binary systems, e.g. AT Microscopii, which consists of two dM4.5e stars orbiting around each other. 15

16 Visible-light spectrum of AT Microscopii (dMe,UV Ceti star) 16 HβHβ HγHγ HδHδ Ca II H & K TiO band-heads From SAAO 1.9 m telescope + spectrograph: 1980 From Bromage et al. (1982) Wavelength in Å (1Å = 0.1 nm)

17 X-ray and ultraviolet emission from stars 17 Spacecraft from the 1970s showed that many stars emit ultraviolet and X-ray radiation. Many of these stars are similar to the Sun, having late spectral types (F, G, K, or M), with effective temperatures ranging from 7400 K or cooler: M5 stars have T eff = 2800 K. Their UV spectra show emission lines like C IV (155 nm, emitted at ~10 5 K), He II (30.4 mm, 10 4 K). The most intense X-ray emitters are also characterized by large flares, with total flare emission >> the emission from the star itself.

18 UV & visible-light spectrum of AT Mic 18 UV spectrum from IUE satelliteSAAO 1.9m tel. spectrum 1Å = 0.1 nm

19 Short history of Flare Stars Flare-like brightenings observed optically on certain cool M-type dwarfs: DH Carinae (in 1924) and UV Ceti (in 1948). These cool stars were also characterized by emission lines in their visible-light spectra, particularly the Ca II H & K lines and Hα lines. Observations in late 1950s with the Jodrell Bank radio telescope by Lovell and others showed large increases in m-wave radio emission from cool stars including UV Ceti. 19

20 Stellar activity and link to magnetic field Stellar activity defined by presence of strong chromosphere and corona, and the occurrence of active regions and flares (probably CMEs too). Stellar activity must be linked to presence of magnetic fields which convey energy from the interior to the atmosphere by either MHD waves or tiny (nano-) flares. The magnetic field is generated by differential rotation and by convection. So stellar activity has two ingredients: fast stellar rotation and convection. 20

21 Differential rotation and convection 21 Differential rotation over a solar cycle (Babcock model). Convection current carries magnetic field upwards while Coriolis forces twist field (sense shown is for S hemisphere).

22 Energy transport in stars as a function of T eff For very hot main sequence stars, whole of stellar interior is fully ionized and energy transport is entirely by radiation. By spectral class F0 (T eff = 7400K), H is ionized just beneath the stellar surface: radiative transport occurs up to region near surface, then convective transport occurs. For progressively later (cooler) types (G, K, M), convection becomes increasingly important. For M stars with T eff < 3500K, whole of star may be convective. 22

23 Stellar activity and spectral type X-ray coronae and flares are evident in main sequence stars with spectral type of F0 and later. Very cool stars (M stars of the UV Ceti type) have the most pronounced flaring activity = flare stars. The most active dM and dK stars are those with rapid rotation rates, which suggest youth (i.e. just formed from gas cloud – older stars lose their angular momentum via stellar winds). Some B and A stars may also have flares, as do stars with special characteristics, generally binary systems. 23

24 HR diagram for nearby stars MS stars liable to have coronae RS, XC NC RS = RS CVn binaries. XC = X-ray coronae NC = no coronae, just cool winds dM stars (incl. some dMe stars)

25 Line source functions Whether lines are in emission or absorption depends on the source function: S = j/κ = emission coefft./absorption coefft. = processes that create line photons / processes that destroy line photons. These processes are due to (a) collisions of the emitting atoms with electrons; (b) excitation or de-excitation of emitting atoms with photons from the photospheric radiation field. If there is a chromosphere, electron collisions become very important. For flare stars with their dim, red photospheres, the radiation field is not so important. 25

26 Collisionally and photoelectrically controlled lines in solar and stellar spectra In the Sun, photospheric radiation field is quite strong, so many lines (like Hα and other Balmer lines) are photoelectrically controlled, collisions of electrons not important. Case of Ca II H and K lines slightly different: the chromosphere gives rise to emission line cores since there the lines are collisionally controlled. But for flare stars, there is a dense chromosphere with much higher temperature (~10,000K) than the photosphere (~3,000K). This results in nearly all lines (Ca II H and K, Balmer lines (Hα, H β etc.) being collisionally controlled, with strong emission peaks formed in the stellar chromosphere. 26

27 Stellar flares 27 Flare on Kruger 60B – Kruger 60 is a binary system with two dM stars. Kruger 60B is a flare star. Successive exposures (a to d) are 2 ¼ minutes apart. 60A 60B (a) (b) (c) (d)

28 Flare light-curve (U magnitude) on EV Lacertae 28 Roizman & Shevchenko (1982)

29 Balmer line spectrum from flare on AD Leonis (UV Ceti star) 29 Spectrum shown is difference of flare and pre-flare spectrum. Balmer lines + Ca II H,K lines + He I (402.6nm) lines all visible. Note 1 Å = 0.1 nm.

30 Case of AB Doradus (AB Dor) AB Dor is a young, rapidly rotating dK star – period of rotation is 12 hours (cf. Sun days). Rotation velocity at equator >80 km/s (cf. Sun 2 km/s). The Einstein X-ray spacecraft found X-ray flares. Rotational modulation in visible-light, suggesting giant starspots. Strong radio emitter. Variable absorption features in Hα line profile attributed to huge prominences (Collier Cameron). 30

31 Doppler imaging of a stellar prominence: Hα line profile 31 Hα line centre is at nm = 6563 Å Velocity from v = Δλ/c

32 Doppler imaging of a stellar prominence 32 Observations of AB Doradus by A. C. Cameron in 1986.

33 Properties of the clouds in AB Dors spectrum Observations suggest that clouds are like prominences (temperatures 10,000K, electron densities around m -3 ). Prominences appear to co-rotate with star, i.e. revolve around the star in a 12-hour period. If so, they are probably held in giant magnetic loops extending out to 2.5 x the stars radius. 33

34 Other active stars: non-solar stars Very young – T Tauri – stars forming out of a hydrogen cloud often show flaring activity. They appear to be only a few 10 6 years old. Flares probably result from blobs of material that cascade into the newly formed star from an accretion disk. A powerful stellar wind is driven from the main star as it continues to collapse. The Sun passed through this stage early on its lifetime, with giant flares and solar wind. 34

35 Model for T Tauri stars 35 T Tauri star forms out of gas cloud. Accretion disk supplies mass to the forming star. X-ray flares result from blobs of material falling into star from disk. A powerful stellar wind flows from the main star.

36 T Tauri and Hinds nebula T Tauri is the yellow star in this image, with a nebula (NGC 1555) near by. Both vary with time but not simultaneously. (APOD ) 36

37 W Ursae Majoris (W UMa) stars Binary star system consisting of two dwarf stars. The two stars are in contact with each other as they revolve. Material is transferred from the more massive star to its companion. Orbital periods are < 1 day. Ultraviolet and X-ray observations suggest that the stars share a common chromosphere and corona. 37

38 Schematic of W UMa system 38 Two stars orbit each other in ~ 1 day; mass transfer from more massive to less massive star. Stars share chromosphere/corona.

39 RS Canum Venaticorum (RS CVn) stars Binary star systems in which an F or G main sequence star orbits an evolved K subgiant star. The two stars are tidally locked (rotation period of each star = their orbital period). Magnetic loop structures may connect the two stars. Very hot coronae, with temperatures ~50-80 MK. Giant X-ray flares may occur. 39

40 Scheme for RS CVn star system 40 Stars are separate while orbiting each other. Mass transfer from K subgiant to G main sequence star. Corona made up of magnetic loop systems connecting the stars.

41 Doppler imaging of starspots Optical light-curve of some active stars show modulation with same period as the rotation period of the star. Prominent absorption lines (like Hα) may show extra absorption features which cross the line profile. This can be used to map the stars surface, the extra absorption features being dark areas on the stars surface. This Doppler imaging technique has been used to search for starspots. Possibility that starspot imaging is confused by presence of planets around stars which have a similar modulation pattern (which the Kepler spacecraft is searching for). 41

42 Doppler imaging of starspots: HR1099 (one component of an RS CVn system) 42 Observations by S. Vogt & A. Hatzer in 1985

43 Summary of active stars UV Ceti stars: single (or members of distant binary stars) showing emission lines in UV and optical regions: evidence of chromospheres/transition regions/coronae. Also strong flares. Like the Sun only more so. T Tauri stars: newly born stars, with accretion disks. Frequent flares arising from blobs of material falling into stars. The Sun had a T Tauri phase early on. W UMa stars: contact binaries consisting of dwarf stars, with more massive star transferring mass to other star; share chromospheres, coronae. Strong flaring occurs. RS CVn stars: F or G main sequence star orbiting an evolved K subgiant star with mass transferred to its companion; magnetic loops extend out from each star. Giant flares. Coronal temps. ~50-80 MK. 43

44 The Suns early history We saw how the Sun formed from the collapse of a large cloud of gas. Most probably, several stars formed out of a gas cloud with mass many times M ʘ -- perhaps 1500 – 3500 M ʘ. When we look out now (4.6 Gyr after the Sun was formed), there are only a few bright stars that are nearer to the Sun than 10 pc (e.g. Sirius, α Cen, Altair), but lots of very faint dM stars. 44

45 Neighbours of the Sun 45 Courtesy: Scientific American

46 The Suns siblings: where are they now? So what happened to the ~ stars (assuming average stellar mass = 1 M ʘ )? The Sun is located about 10 kpc from the centre of the Milky Way Galaxy (galaxy diameter ~ 30 kpc). Its orbital speed is about 234 km/s, and period ~ 270 x 10 6 yrs = 0.27 Gyr. So in its 4.6 Gyr lifetime, the Sun will have orbited the Galaxy about 20 times. In this time, the original star cluster that was formed has spread out into an arc centred on the Galaxys centre 46

47 Milky Way Galaxy Scheme of the Milky Way galaxy with spiral arms and barred centre. The Sun is 10 kpc from the Galactic Centre, and the whole galaxy has a diameter of 30 kpc. 47 SUN GALACTIC CENTRE 30 kpc

48 Intriguing possibility The Suns fellow cluster members are probably still around, but only ~400 are probably within 1 kpc of the Sun: they could be identified with the ESA Hipparchos spacecraft. Some meteorites recently analyzed, thought to be leftovers from the formation of the solar system, have Ni 60 in them, which can only have been formed from Fe 60 within the past 3 Myr. The Fe 60 must have been formed, and injected into the meteorites, in < 3Myr. 48

49 A supernova near the Sun? Where did this material come from? Speculation is that one of the original star cluster members had a large mass and underwent a supernova explosion, distributing the material into the vicinity of the Sun when the Sun was only 2 Myr old. The supernova might have been only a fraction of 1 pc away. 49

50 Giant flaring on the early Sun? A survey of very young stars in the Orion Association shows that Sun-like stars can undergo giant flares (total energy >> J). Did this happen for the young Sun, i.e. when it was in its T Tauri phase? It might conceivably explain why the Sun has a very low abundance of Li (which was mostly formed in the Big Bang), only 1/150 times the cosmic abundance. Giant flares might have resulted in nuclear reactions that in time broke down Li into H or He. 50

51 Concluding Remarks Several nearby stars have properties like the Suns in terms of activity: coronae, chromospheres, active regions, starspots, prominences, and stellar flares. Some stellar flares are much more energetic than solar flares. Sun probably formed in a cluster, but original cluster members not very evident. Possibility that a supernova went off early in the Suns history, and that giant flares occurred. 51

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