Standard Candles in Astronomy

Contents

A standard candle is a class of astrophysical objects, such as supernovae or variable stars, which have known luminosity due to some characteristic quality possessed by the entire class of objects. Thus, if an extremely distant object can be identified as a standard candle then the absolute magnitude M (luminosity) of that object is known. Knowing the absolute magnitude, the distance D (in cm) can be calculated from the apparent magnitude m as shown in the formula below.

m = M - 97.5 + 5xlog(D) ---------- (1)

Figure 01 below shows the different kind of "standard Candles", including the Cepheid Variables (star), the planetary nebulae, and the Type Ia supernovae (together with other distance measuring tools).

Figure 01 Standard Candles

Parallax - The parallax of a star is the angle subtended at the star by the radius of the Earth's orbit (Figure 02). Distance to the star can be computed by the formula: d = 206265/p where the distance d is in unit of a.u., and the parallax p in second of arc. The usefulness of this method is limited by the resolution of the apparent positions to a distance of about 100 light years. A parsec is the distance corresponds to a parallax of 1 sec. of arc, i.e., 1 parsec = 206265 a.u. = 3.26 light years.

Figure 02 Parallax [view large image]

• Cepheid Variables - It was discovered in 1912 by the American astronomer Henrietta Leavitt that the period of the Cepheid variables depends on the star's mean absolute magnitude; the more luminous the star, the longer the period

  		(Figure 03a). From a graph or a formula it is possible to find the absolute magnitude of a Cepheid variable if its period is known. The distance to the star D can then be computed from Eq.(1). The Cepheid variables have proved extremely important for astronomy because they can be used to calculate the distance of objects far beyond the Milky Way. The same kind of relation is also applicable to the RR Lyrae stars. They are all about fifty times more luminous than the Sun, and they vary by about a magnitude in periods of between 0.2 and 1.2 days. Figure 03b shows the Cepheid Variable star RS Pup (behind the dark central stripe) surrounded by
  Figure 03a Period-Luminosity Relation [view large image]	Figure 03b Cepheid Star RS Pup [view large image]	dusty reflection nebula. It is some 10 times more massive than the Sun and pulsates over a period of about 40 days.

  
  

  Planetary Nebula Luminosity Function - It is found that the statistical distribution of planetary nebula brightness in a galaxy obeys the so called planetary nebula luminosity function (PNLF) below: N(M) ~ e0.307M{1 - e3(M* - M)} ---------- (2) where N is the number of planetary nebula with absolute magnitude M, and M* represents the cutoff magnitude (there are no PN brighter than this), which is about -4.48. Thus, if we have a sample of PN from a given galaxy satisfying the PNLF

  Figure 04 PNLF [view large image]

  as shown in Eq.(2), then we can obtain the apparent magnitude m* corresponding to the cutoff magnitude M*, and the distance D can be computed from Eq.(1). Figure 04 shows the theoretical and observed PNLF for M31.

  
  
• Tully-Fisher Relation - In stable orbit, the centripetal force of an object (be it a star or gaseous medium in the spiral arm) moves around a galaxy with mass M and rotational velocity V is balanced by the gravitational force such that:
  
  mV² / R = GmM / R² --------- (3)
  
  where m is the mass of the object. Assuming all galaxies have the same mass luminosity ratio (M/L constant) and the same surface brightness, i.e., I = constant = L /(4R²), then it follows from Eq.(3) that the luminosity L V⁴. Then the

  absolute magnitude M can be expressed in term of V, which can be measured from the line width of the 21-centimeter line emitted by the hydrogen cloud. This is known as the Tully-Fisher relation as shown in Eq.(4): M = 4.8 - 2.5xlog(L/Lsun) ~ 10xlog(V) ---------- (4) The distance D can be calculated by Eq.(1) once the apparent magnitude m of the galaxy is known. Figure 05 is an example of a Tully-Fisher relation between M and log(V). Each data point corresponds to a galaxy within the cluster of galaxies. The solid line represents the theoretical best fit.

  Figure 05 Tully-Fisher Relation [view large image]

• VLBI - Over the last decade, astronomers have been developing new techniques that will allow them to skip the distance ladder altogether - one of them is called Very Long Baseline Interferometry. Using the Very Long Baseline Array (VLBA) - a set of ten radio telescopes situated around the world from Hawaii to the Virgin Islands - astronomers can make direct distance measurement to spiral galaxies such as M106 (Figure 06). With the radio telescopes acting as one giant dish thousands of miles wide, they were able to tightly focus on the galaxy¹s core and measure the velocity of water masers - compact molecular clouds that emit an intense radio signal - swirling in a disk of gas at the galaxy¹s

  	center. Combining the maser's intrinsic speed (inferred from Doppler shift of the maser emission) with the apparent angular motion as seen from Earth yields the distance D via the formula: D = V / (d/dt) ---------- (5) where V is the intrinsic velocity of the H2O molecular cloud measured from Doppler shift of the maser emission, and d/dt is the proper motion of the molecular cloud
  Figure 06 M106 [view large image]	observed directly by the VLBI. Sometimes the proper motion of the jet is measured (instead of the molecular cloud) such as in the case of 3C273.

• TypeIa Supernovae - Type Ia supernova is the explosion of a white dwarf star in a binary star system. Material from a companion red giant star is dumped on the white dwarf until the smaller star reaches a precise mass limit. At that point the white dwarf can no longer support its own weight, and burns its nuclear fuel so suddenly that it explodes. These explosions always release roughly the same amount of energy, and studies of relatively nearby type Ia supernovae have

  		shown that they reach almost the same peak brightness in every case. Therefore it can be used as "standard candles" to determine their true distance. Figure 01 is a Type Ia supernova observed in 1994. It is the bright spot on the lower left at the fringe of the galaxy. Figure 02 shows such binary system before the explosion.
  Figure 01 Supernova Type Ia	Figure 02 Binary System [view large image]	The absolute magnitude for the Type Ia supernovae has been calibrated to be M = -19.33 0.25

• Sunyaev-Zeldovich Effect - The clusters of galaxies contains a hot, diffuse, gaseous intergalactic medium. The gas emits x-rays through the bremsstrahlung process. The x-ray luminosity is proportional to ²S, where is the density of the

gas, and S is the size of the cluster. While the Compton scattering process transfer energy between the gas and the photons of the cosmic microwave background, effectively casting a shadow on the CMBR. The cluster is dimmer when observed at low frequencies but brighter at high frequencies, relative to the CMBR as shown by the dotted line in Figure 09. The magnitude of this effect is proportional to S. The x-ray luminosity and the microwave variation are combined to obtain a direct measurement of the size S of the region containing the hot gas. While direct observation gives the angular size . Thus the distance D can be computed by the formula:

Figure 09 SZ Effect [view large image]

D = S / ---------- (6)

• Gravitationally Lensed Quasars - When a galaxy is very close to the line-of-sight of a quasar, the light is deflected and many images of the quasar appear (Figure 10). As quasars are intrinsically variable objects, monitoring the luminosity variations of all images allows us to measure time delays between images, which is caused by splitting the light into slightly different paths around the lensing galaxy (see Figure 11). Computing the distance from the earth to the quasar requires the following information:

		The time delay -- which means the intensity of the background source must vary sharply, by large amounts. The angular separation of the images of the background source. the relative distances of lens and background source (we can use the redshift for this). the mass of the lens, and its distribution (a point mass, or uniform sphere, or a sphere with varying density, or an ellipsoid).
Figure 10 Lensed Quasar [view large image]	Figure 11 Time Delay [view large image]