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Home , 3 Centauri, Absolute magnitude, Apparent magnitude, Black dwarf, Centaurus, HR 5171

1 2018 Div. C (High School) Help Session Sunday, Feb. 18th, 2018 and Type II supernovae Scott Jackson Mt. Cuba Astronomical Observatory

• SO competition on March 3rd . • Resources – two computers or two 3 ring binder or one laptop plus one 3 ring binder – Programmable calculator – Connection to the internet is not allowed! – Help session before competition at Mt. Cuba Astronomical Observatory 2 3 Study aid -1 • Google each object, – Know what they look like in different parts of the spectrum. For example, the IR, optical, UV and Xray – Understand what each part of the spectrum means – Have a good qualitative feel for what the object is doing or has done within the astrophysical concepts that the student is being asked to know. 4 Study aid - 2 • Know the algebra behind the physics – Just because you think you have the right “equation” to use does not mean you know how to use it!!! – Hint for math problems: Solve equations symbolically BEFORE you put in numbers. Things tend to cancel out including parameters you do not need to have values for. – Know how to use scientific notation. 5 The test – 2 parts • Part 1 – multiple choice and a couple fill in the blanks

• Part 2 – word problems for there will be some algebra Solve the equations symbolically first then put in numbers!!!! Hint: most problems will not need a calculator if done this way Topics - 1 Stellar evolution, including - , - spectral features and chemical composition, - H-R diagram transitions, - disks - - HII regions - red supergiant giants, - Cepheids - Semiregular variales - Luminous blue variables - , Wolf-Rayet stars, stars - , , black holes - Eclipsing binaries, X-ray and -ray binary systems - Type II Kepler's laws as they apply to binary systems, Distance latter in the universe, calculating distance and Know about specific objects 7 8 RCW 103, - supernovae remnant with young – slow or binary IC 443 – Jelly fish nebulae from a explosion Alpha Orionis – The brightest star in the , RSG HR 5171 A V766 Centauri, a star in a triple aout 12,000 ly away SN W49B a in the constellation ASASSN-15lh, Supernova SN-15lh – likely or a from a magnatar AG Carinae, – Star in Constellation . Shedding mass at huge rate, LBV , - Bright star in the Large Magellenic Cloud LBV SN 1987A, SuperNova that occurred in 1987 in the large Magellenic Cloud , -Rapidly rotating – pulsar in NGC 6357– Diffuse in . Many new stars: OB association NGC 7822, Star forming region in : OB association M82 X-2, Xray pulsar in the M82. PSR B0355+54 Pulsar in the constellation DEM L241, Supernova remnant in the X-1 X-ray binary system in Circinus RCW 103, -Supernova remnant, contains a supernova remnant that is a 9 magnetar (Neutron star with powerful magnetic fields and a very slow pulsar (rotating neutron star in this case rotating once every 6.5 hours – pulsars usually take less than a second to rotate) - Other possibility: A binary system with a companion orbiting a normal pulsar every 6.5 hours - Located about 9000 ly ( ) from . - Magnetar is only one of 30 known – age estimated to be 2000 years old – too young for pulsar to slow down to a period of 6.5 hours IC 443 Jellyfish Nebula – 5000 ly from earth 10 Contains a pulsar with a jet and a ring Alpha Orionis High mass star, several million years old, at the 11 end of its life. Expected to explode as a Type II supernova “soon” In the red supergiant (RSG) stage now. 12 to 17 solar masses. First star to have its surface imaged. Tsurface ~ 3400 K. Star is 4.5 au in diameter -- Would almost go out to It is surrounded (right) by a large (400 au in diaeter) nebula of gas and dust. HR 5171 A in the constellation , around 12,000 light 12 years from Earth. It is either a red supergiant or recent post-red supergiant , and one of the largest known stars. 12,000 ly away. May be 1,300 times the diameter of the 50% larger than Betelgeuse Part of a binary system with the companion believed to be touching the main stars surface

UV light showing tail 13 SN W49B Type II Supernova remnant May have left behind a back hole and not a pulsar May be the youngest in the Xray . Supernova occurred around 17,000 to 21,000 years ago. Odd nebula caused by material ejected out the poles instead of the equator Composite

Infared 14 ASAS-SN-15lh Supernova discovered using the All-Sky Automated Survey for Supernova. Intrinsically the brightest supernova yet observed – 570 billion x sun. Considered a “hypernova” Z ()=0.2326 [1171 megaparsecs away] 15 AG Carinae, Luminous blue (LBV) came from a star around 50x the mass of the sun. Also known as HD94910 20,000 ly away. May become a Wolf-Rayet star. Loosing a huge amount of mass due to its very strong solar pushing the material way from the star and making the nebula you see surrounding the star

Visible (HST) Radio S Doradus– One of the most luminous stars known. 1 million x 16 the sun’s In constellation of . star. Lies in called NGC 1910. It is variable (below) and is in an (Next slide)

Light curve S Doradus– LBV are variable due to dense solar wind that 17 creates a falsely large star. That solar wind dissipates and the drops Apparent radius of the star changes from 100x to 380 x our sun. Most will eventually become Type II SN

6.5-

6-

Zero Age main sequence

5.5-

Log Luminosity 32,000K 10,000K 3,200K | Log | | 18 SN 1987A, Type II supernova. from the exploding star smashes into a ring of material and caused the ring to brighten. Ring of material was made before the supernova happened – during the time when the star had strong solar winds. Ring is about 1 light in diameter. Ring is 20,000 years old -- in Large Magellanic Cloud 19 Geminga Remnant of a supernova that occurred 300,000 years ago. Pulsar with a period of 0.24 seconds. Originally observed as an unknown source. 250 away (Gemina Gamma-ray source). Very high (speed) Through our galaxy Once believed that a bubble around Our came from Geminga 20 NGC 6357 New star forming region in Scorpius. HII region– ionized region – hydrogen being ionized from strong form the new stars. 5500 light years away.  OB association of stars

Composite X-ray 21 NGC 7822 Star forming region in Cepheus. 800 to 1000 parsecs away. Includes one of the hottest nearby stars known : a massive type O stars – surface temperature of 45000K, luminosity of ~100,000x our sun. another OB association 22 23 M82 X-2 Very bright X ray source in the M82 galaxy in the constellation of Urasa Major. A neutron star comsuming material from an adjacent star. Brightness limited by Radiation pressure balances Gravitational forces

Flow of material to dwarf PSR B0355+54 Pulsar in the constellation Camelopardalis 24 3460 light years away. Period is 0.715 seconds. 5 million years old. Pusar is moving through interstellar media and generating a high energy tails (see in Xrays) by the Chandra xray satellite. https://arxiv.org/pdf/1610.06167.pdf 25 DEM L241, A star that survived a supernova explosion. System contains a neutron star or black hole and a massive companion star. Supernova remnant is DEM L241. IN the large Magellanic Cloud The progenitor star was likey 25x to 40x the mass of the sun.

Visible

X-ray Composite 26 Circinus X-1 X ray (Neutron) star with a second star (binary system) . Shows X-ray jets that are seen in black hole systems. Young X-ray binary system. Less than 4600 years old. In the constellation Circinus ~9400 parces away

radio Visible+Xray+ radio

X-ray 27 Circinus X-1 X ray (Neutron) Visible star

Light “Echos” from outbursts reflected in nebulae Visible+Xray

X-ray 28 Brightness of Stars • Brightness measured as luminosity or – Luminosity is the total energy output of a star • Depends on size and surface temperature • Usually measure relative to our sun, e.g., 4 times our sun. – A star’s magnitude is the of its luminosity – (m) [what we see] – is determined by four factors • Its temperature or color (wattage of a light bulb) • Its size • How far away it is • If it is obscured by dust () – (M) • Magnitude of a star when viewed from a fixed distance • Most abs magnitudes will be a negative number (bright) 29 Brightness of a star: A star’s magnitude

• Magnitude is more often used to describe an objects brightness. • The higher the magnitude the dimmer the object. – The apparent magnitude of our sun is -26.7 – The apparent magnitude of a full is -12.6 – The apparent magnitude of the is ~ -1 – Dimmest star you see (in Wilmington) ~+3.5 – Dimmest star you see in a dark sky location ~+5.5 • The absolute magnitude is the magnitude of the star / object if it was place a fixed distance away (10 parsecs -- later). • The absolute magnitude of our sun is ~ +4.8

30 • O Spectral class of stars • B –SPBs – Slowly Pulsating B type stars - WR: Wolf Rayet stars - DBV: Dwarf B variables

- DAV: Dwarf A variables WR • A • F • G Our sun – G2, M=4.8 • K • M - red SR: SemiRegular • L Red Dwarfs (failed stars) • T Brown Dwarfs (failed stars)

31 Categorizing stars by their spectra

1. Spectra can tell you 2. Absorption (dark) the stars approximate lines in a star’s spectra temperature give a finger print of (blackbody radiation) elements that are seen in that spectral class of stars

BUT emission spectra (bright lines against a dark ) are given off by nebulae – glowing gas clouds

32 Spectral class of stars He+ lines

H Balmer lines (B,A & F stars)

Ca+ lines (F & G stars)

Fe and neural metals K & M stars)

TiO2 lines

33 Spectral classification & Temperature of main sequence stars

Star Surface Star Lifespan Star Mass Spectral Proportion of Stars Temperature Luminosity (Billions Example Star (Sun = 1.0) Class (°F) (Sun = 1.0) of Years) A0 1% A0 - A9 20,000 2.8 60 0.5

A1 --- 18,400 2.35 22 1.0 Sirius A5 --- 15,000 2.2 20 1.0 --- F0 3% F0 - F9 13,000 1.7 6 2.0 --- F5 --- 12,000 1.25 3 4.0 A G0 9% G0 - G9 11,000 1.06 1.3 10 --- Sun G2 --- 10,600 1.00 1.0 12 A G5 --- 10,000 0.92 0.8 15 --- K0 14% K0 - K9 9,000 0.80 0.4 20 Alpha Centauri B K2 --- 8,700 0.76 0.3 24 K5 --- 8,000 0.69 0.1 30 A M0 73% M0 - M9 7,000 0.48 0.02 75 --- M5 --- 5,000 0.20 0.001 200 34 (Alpha Centauri C) 35 More on stars spectral class

36 Y axis is always 37 Hertzsprung-Russell Diagram brightness or relative luinosity X axis is always temperature, color or spectral class Each dot is a star A is the location of our sun on the main sequence B are stars that are fusing in their core C are red L supergiants with T Helium and Hydrogen buring in http://outreach.atnf.csiro.au/education/senior/cosmicengine/stars_hrdiagram.html shells and in D are white dwarfs (super hot carbon stars) its core 38 Instability gaps on an H-R diagram for the pulsating class of variable stars Period of pulses scale with absolute brightness of the star “Period-luminosity relationship”

• http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_pulsating.html 39 Where does the S Doradus instability gap reside? Upper right hand corner!!!

• http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_pulsating.html Accretion disks • Circumstellar disks • Many accretion disks seen in systems when one star hass filled its “Roche” limit and is having material “sucked” away from it to a companion star • HR 5171 A and likely M82 X-2

http://planetquest.jpl.nasa.gov/documents/RdMp272.pdf Birth of a star 42 The birth of a 1 solar mass star going onto the main sequence. Before point 4, contraction of intersteller gas cloud. The cloud heats up as it contracts, causing its luminosity to increase -- we don’t see it because the is hidden in dust. From point 4 to 6, -- The cloud contracts more and its luminosity drops. Point 6, hydrogen starts to fuse to helium in the stars core. The heat generated from fusion balances . The star’s surface heats up slightly. This is the location of stars Point 7. The star has reached a long lived equilibrium where the heat from fusing hydrogen to helium balances gravity. The star resides on the main sequence for most of its life (~10 billion years for a 1 solar mass star). Death of main sequence stars Red Giant for lower mass stars

Low mass star like our sun stops at carbon formation in its core... And fluffs off its outer layers to make a planetary nebulae and a white . Red super Giant for higher mass stars But a high mass star, like those in the early universe had enough mass to fuse nuclear material all the way to . However, once iron accumulates in its core no net energy generation can be done by fusion of iron, gravity takes over and core collapse occurs and..... are pushed into making and a flood of …. It goes boom!!!!... A supernovae!!! (this is the Crab Nebulae) … Which make lots of heavy elements needed to make terrestrial (earth like) . This is NOT a type 1a supernovae. It is a type II supernovae. .. And it spreads heavy elements throughout space to be picked up by a new generation of stars,...... The shock wave either from the supernovae or from the initial stage can initiate new star formation,..... 50 Before they ultimately die, high mass stars go through a red supergiant stage During this time they may have very strong “solar wind” and shed a lot of its mass. – like AG Carinae and Betelgeuse The strong solar wind may also make the star appear larger (and hence brighter) than normal – like S Doradus --LBV Stars and planets approximate radiators The wavelength at maximum radiation changes with temperature

λmax = 550 nm  5300 K temperature for our sun. “G” type star (subclass “2”) or G2

λmax x Temperature = constant = 2.9x106 nm-°K Or = 2.9x107 A-°K = 2.9x103 μm-K Nm[=] nanometers for wavelength Or A [=] Angstrom units for wavelength Or μm [=] microns units for wavelength °K [=] degrees 51 Another way to look at black body radiation

Plot log λ (x axis) vs log of spectral intensity at that λ Example calculation for a star’s temperature

So the shorter the wavelength the hotter or colder the star????

λmax ~ 0.9 μm What it the star’s temperature? T ~ 2.9x103 μm-K / 0.9 μm = 3200 K (M type star)

If λmax ~ 10 μm What it the star’s temperature? λmax x Temperature = constant = 2.9x106 nm-°K T ~ 2.9x103 μm-K / 10 μm Or = 2.9x107 A-°K = 2.9x103 μm-K = 290 K () Nm[=] nanometers for wavelength Or A [=] Angstrom units Or μm [=] microns units °K [=] degrees Kelvin 53 • What is the “temperature” of an object emmitting x-rays or gamma rays?  * temperature = 2.9x106 nm-°K If  is 1 nm (soft xrays) then Temperature = 2.9 million degrees(!)

What is max for an “O” type star? T~ 40,000 K  max ~72 nm  it shines in the light!!! We can still “see” it in visible light because part of its light is there. What is max for Betelgeuse? T~3400K  max ~ 850 nm or 0.85 mm

54 Neutron stars • When higher mass stars “die” gravity takes over and the core of the star collapses. degeneracy pressure is overcome and electrons are pushed into the protons to form neutrons (and a flood of neutrinos – that give rise to a supernovae). • Initial will be distributed between the supernovae remnant and the resulting neutron “star”. • The angular momentum of the neutron star can cause it to spin very quickly – creating a pulsar. • Strong magnetic fields can focus a beam of radiation like a light house • Pulsars can have an (from the blown off remnant of the star) that generates x-rays as matter is accelerated to near the as it falls into the

neutron star. 55 Mass of the main sequence star is reduced as it evolves and dies.

Material is shed either during the formation of a planetary nebulea () or during a supernovae.

The supernovae in this diagram are meant to be Type II and not Type Ia.

56 Topics - 2 HII regions Luminous Blue variables Wolf-Reyet stars Hypergiants, Magnetars

Use Kepler’s laws of rotation and circular motion to answer questions relating to the orbital motions of binary systems; use , , type Ia SN and the distance modulus and Hubble’s law to calculate distances. HII regions Generally star forming regions in the galaxy where hot new stars have ionize hydrogen causing it to “glow” . NGC 6357 and NGC 7822

Luminous Blue Variables (LBV), Red supergiants (RSG) and Wolf-Rayet stars (WR). Evolved from Main Sequence OB stars [O type stars or early B type stars]. Very massive stars that are formed in groups call OB associations. Lots of UV radiation emitted. High mass Slower spin Main Sequence Fast spin & mass loss & mass loss RSG O stars

WR LBV S Doradus variables Type II SN 58 Wolf Rayet stars Hot massive stars at the end of their life – shows broad emmission lins of ionized helium, or carbon. Strong stellar winds, Surface 30,000 K to 200,000K

Main SequenceSpectra of a Slower spin W-R star & mass loss RSG

Type II SN 59 Magnetars Neutron star with a very strong . X-ray and gamma ray emmisions. RCW 103

Hypergiants Extremely high luminosity and high mass loss caused by stellar winds. Largest stars known. HR 5171 A may be a hypergiant star

60 Kepler’s laws – gold standard for “weighing” stars

1. are ellipses with sun at one focus 2. Equal areas swept out in equal time 3. Harmonic law: Square of the period (P) is proportional to the cube of the semimajor axis (a). -- Gold standard for determining masses in the universe – and binary stars. Kepler’s law 2 3 P = a / (m1 + m2) P = (years) a = Distance between the two bodies (expressed in astronmical units [AU] – distance from earth to sun) 1 AU = 107.5 sun diameters or 215 sun’s radius m1, m2 = mass of the two bodies orbiting each other (solar masses)

61 Measuring Distances…

Brightness of stars…

62 Brightness of Stars • Brightness measured as luminosity or magnitude – Luminosity is the total energy output of a star • Depends on size and surface temperature • Usually measure relative to our sun, e.g., 4 times our sun. – A star’s magnitude is the logarithm of its luminosity – Apparent magnitude (m) [what we see] – is determined by four factors • Its temperature or color (wattage of a light bulb) • Its size • How far away it is • If it is obscured by dust (extinction) – Absolute magnitude (M) • Magnitude of a star when viewed from a fixed distance • Most abs magnitudes will be a negative number (bright) 63 Brightness of a star: A star’s magnitude

• Magnitude is more often used to describe an objects brightness. • The higher the magnitude the dimmer the object. – The apparent magnitude of our sun is -26.7 – The apparent magnitude of a is -12.6 – The apparent magnitude of the Sirius is ~ -1 – Dimmest star you see (in Wilmington) ~+3.5 – Dimmest star you see in a dark sky location ~+5.5 • The absolute magnitude is the magnitude of the star / object if it was place a fixed distance away (10 parsecs -- later). • The absolute magnitude of our sun is ~ +4.8

64 65 Distances • . Average distance between the earth and our sun. (AU = 1.496x1011 meters or 97 million miles or about 8.3 light minutes) This is a small unit of measure. – Used for interplanetary measures and for distances between stars in binary star systems (Kepler’s Laws) • Light years. The distance light travels in a year – LY = 9.46x1015 meters, 6.33x104 AU • [pc]. The distance to an object that has a parallax of 1 arc second (next slide)  preferred unit by pc = 3.26 LY = 2.06x105 AU = 3.086x1016 meters • Kiloparsecs (Kpc)  1000 parsecs (103 parsecs) • Megaparsecs (Mpc)  1 million parsecs (106 parsecs)

66 67 • Geometric parallax  Gold standard for distances – 1 Parsec = 3.09 × 1016 meters • parsec - (pc): distance at which an object would have a parallax of one arc second. Equals approximately 3.26 light years or about 206,265 astronomical units

Star appears to move with season Don’t move 68 69 Spectroscopic Parallax 1. Measure the spectrum of a star. Lines in the spectra will indicate if it is a main sequence star . The star needs to be bright enough to provide a measurable spectrum, which is about 10 000 parsecs. 2. Using the star spectra or using the UVB index, make certain that it is on the main sequence, deduce its spectral type (O, B, A, F, G, K, M, L) 3. From the spectral type deduce its absolute magnitude [M] (H-R diagram or table) 4. Measure the apparent magnitude (m). Knowing the apparent magnitude (m) and absolute magnitude (M) of the star, one can calculate the distance modulus (m-M) and the actual distance in parsecs – next slide. Good for stars that are <~ 10,000 parsecs from us (or 32,600 light years) – most of the stars in our galaxy. 70  Distance modulus is m-M if there is no interstellar dust (or extinction)  If there is interstellar dust then distance modulus is ((m-E)-M) where E is the extinction magnitude

The larger the distance modulus the further away the object is. Little m is usually >+10 Capital M is usually small – many times negative, E can be as much a 1 or 2 (magnitudes of extinction due to dust in our galaxy) 71 Relationships between distance modulus, luminosity, distances in parsecs and absolute magnitude

Msun = 4.8 (absolute magnitude or our sun)

Astronomical unit [AU] = average earth- sun distance 1 AU = 1.496 x 108 km Diameter of our sun = 1.391 x106 km 1AU = 107.5 sun diameters What is distance modulus for our sun? 72 73 Instability gaps on an H-R diagram for the pulsating class of variable stars Period of pulses scale with absolute brightness of the star “Period-luminosity relationship”

• http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_pulsating.html 74 Period-Luminosity Relationship equation for type 1 Cepheid

For Type I, Type II Cepheids and RR Lyrae Cepheids named after the first star discovered in the constellation Cepheus (up north) M = -2.81* log(P)-1.43 Note this is luminosity – these stars are much P is period in days brighter than our sun.

• http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_cepheids.html

75 for

• Saw tooth curve for Type 1

76 RR Lyrae and Cepheid stars as standard candles  Find the period.  This gives the luminosity or its absolute magnitude  Measure the apparent magnitude.  Determine the distance from the apparent and absolute magnitude (distance modulus) (and an estimate of the extinction [E]) The same applies to RR Lyrae variable stars. Once you know that a star is an RR Lyrae variable (eg. from the shape of its light curve), then you know its luminosity M = -2.81* log(P)-1.43 Type 1. P is period in days

77 78 Type Ia supernovae A occurs in binary stellar system (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a to another white dwarf. OR it can be a merger of two white dwarfs.

Material is drawn off the other star (filling its “Roche” limit) onto the white dwarf until the white dwarf reaches the . Then electron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, its core will reach the ignition temperature for carbon fusion as it approaches the limit. If the white dwarf merges with another white dwarf, it will momentarily exceed the limit and begin to collapse, again raising its temperature past the ignition point. Within a few seconds of initiation of nuclear fusion, a runaway reaction will occur and thus causing the supernovae

 Bottom line: Type 1a SN produce a consistent peak in absolute luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. Absolute magnitude is M ~ -19.5 (negative) Type Ia supernovae is where a white dwarf Because the type 1a “blows collapses because it up” at the same mass limit has pulled too much (see earlier discussion) (Chandrasekhar limit ~1.4x material from a mass of our sun) they have nearby companion about the same absolute star onto itself. magnitude at its peak brightness  Standard candle

80 Using Type Ia supernovae as a standard candle • Because a type Ia “explodes” at the Chandrasekhar limit, all type Ia SN are about the same brightness – Type 1a have an absolute magnitude of about M~ -19.5 (that is a negative sign) • Observed in distant . • Observe a supernovae as it occurs, • Construct its light curve • From the light curve determine if it is a type 1a and estimate is maximum apparent magnitude (m) • Distance modulus is then (m+19.5) for Type Ia supernovae (m is apparent magnitude)

81 82 Red shifting a star’s spectrum

Wavelength of light (nanometers, nm)

1 nm = 1x10-9 meters

Increasing red shift 83 Hubble’s law (measurement to very distant galaxies) Fundamental parameter  measure of the expansion of our universe

Hubble’s Law: d = Vr or for small distances d = z * c (z < 0.5)

Ho Ho d = distance in megaparsecs (millions of parsecs) Vr is recessional velocity (km/sec) Measure using red shift of the light spectrum of a galaxy

Ho is Hubble’s constant, ~75 km/sec / megaparsecs z is the red shift = wavelength of the observed light -1 wavelength of the emitted light

C is the speed of light (3x 105 km/sec)

Problem: if wavelength of the observed light is 440 nm and the wavelength of the emitted light is 400 nm

What is Z? What is recessional velocity? What is the distance using Hubble’s law? In mpc? In light years? 84 Answer to problem z = 440 -1 = 1.1 -1 = 0.1 400 Vr = 0.1 x 3x 105 (km/sec) = 3x 104 (km/sec)

What is the distance using Hubble’s law? D = 3x 104 km/sec / (75 km/sec/mpc [kilometers/second/megaparces]) = 3/7.5 x 103 megaparces (mpc) = 0.4 x 103 mpc = 400 mpc = 3.26 light year / pc x 106 pc/mpc x 400 mpc = 1304 x 106 light years or = 1.3 x 109 ly

85 • If the apparent magnitude of a star is +7 and it has a parallex of 0.01 arc seconds, what is its luminosity relative to our sun?

• What is the mass of the star in number of ?

86 More info… An star’s is named using its constellation and letter of multiple letter designation. So… RY Sagittarii is in the constellation Sagittarius (summer sky) and counting up using the alphabet (a, b, c, d, e… z, AA, AB,…. ) it is star RY in this constellation. A class of stars (like the Cepheid variables or RR Lyrae variables) are named after the first star discovered in that class of stars. So the first Cepheid variable was discovered in the constellation of Cepheus. The RR Lyrae variables are named after the RR Lyrae (in the constellation of [string instrument]). The T Tauri stars were named after T Tauri (a star in ). 87 88