The Shapley - Curtis Debate in 1920

In the debate, Shapley and Curtis truly argued over the "Scale of the Universe," as the debate's title suggests. [Picture] Harlow Shapley (left) and Heber D. Curtis (right) Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as "spiral nebulae". Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy. In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. Although the fine points of the debate were more numerous and more complicated, each scientist disagreed with the other on these crucial points. A partial resolution of the debate came in the mid-1920's. Using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy (M31) . These stars allowed Hubble to show that the distance to M31 was greater than even Shapley's proposed extent of our Milky Way galaxy. Therefore M31 was a galaxy much like our own. In the 1930s, the further discovery of interstellar absorption combined with an increased understanding of the distances and distribution of globular clusters ultimately led to the acceptance that the size of our Milky Way Galaxy had indeed been seriously underestimated and that the Sun was not close to the center. Therefore, Shapley was proved more correct about the size of our Galaxy and the Sun's location in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that "spiral nebulae" were indeed galaxies just like our own. Another reason the 'Great Debate' is important is captured nicely in the book Shu, F., 1982, The Physical Universe, An Introduction to Astronomy, (University Science Books, Mill Valley, California) p. 286: "The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes research at the frontiers of science."

The Distance to Andromeda Galaxy (M31)
A small fraction of stars have brightness variations that are periodic due to "radial oscillations" (pulsations which cause expansion and contraction). These are stars which have evolved off the main-sequence (post main-sequence stars). There are two types of (radial) pulsating stars, RR Lyrae and Cepheid Variables, depending on initial masses of stars. Although the periods from 0.5 to 100 days, any given star has a constant period.
RR Lyrae Variables Horizontal branch stars (because of where they appear in the H-R diagram) Periods: ~ 12 to 24 hours Luminosity: ~ 50 Lsun Found in Globular clusters (Pop II stars) Luminosity is independent of period	Cepheid Variables Named after delta Cephei (first discovered) Red Giants and Supergiants Periods: ~ 1 to 100 days Luminosity is a function of period Period-Luminosity relation discovered by Henrietta Leavitt in 1908. There are two types (labelled Type I and II Cepheids) Type I Cepheids a.k.a. Classical Cepheids Luminosity: 400 to 20,000 Lsun Location: Open clusters and the galactic disk (Pop I stars) Type II Cepheids a.k.a. W Virginis Stars Luminosity: 100 to 5,000 Lsun Location: Globular clusters (Pop II stars)
The relationship between periods and luminosities of pulsating stars gives us an important tool to determine distances to external galaxies. Measured Period gives: Luminosity (i.e. Mv, absolute magnitude) Mv = -[2.76 x (log10P - 1.0)] - 4.16 Measure mv (apparent magnitude) Mv and mv => distance from distance modulus eq. mv - Mv = 5 log10d - 5 Edwin Hubble discovered Cepheid variables in M31 (Andromeda Galaxy) and determined the distance to M31, using the Period-Luminosity Relation for Cepheids. The distance to M31 was much larger compared to distances to stellar cluster (since he did not distinguish between type I and type II, he obtained a smaller distance than the actual distance to M31 but still M31 seemed to be located farther away than stellar cluster) and the physical size was comparable to that of Our Galaxy, i.e. M31 finally turned out to be a galaxy, an "Island Universe".

Galaxies and Their Morphological Classification
Our own Galaxy, the Milky Way, is a vast collection of billions of stars, stretching roughly 100,000 light years from end to end. As well, it contains gas and dust along with associated starlight, magnetic fields and cosmic rays (recall, the multiwavelegth features of Milky Way). Despite its enormous size, it is only one of hundreds of billions of galaxies in the known universe. These multitudes of galaxies come in a variety different shapes and sizes. In this exercise, we'll take the first steps in learning about these many different types of galaxies. To help us understand the complexity of galaxies, astronomers have developed several schemes to classify them into broad groups based on their morphological structure. One of the first, and simplest classification schemes for galaxies was developed by Edwin Hubble (1889-1953), a prominent astronomer from the Mount Wilson observatory whose lifetime of work on galaxies is crucial to our understanding of the universe. Hubble seperated galaxies into three broad groups, based upon their morphology . He classified galaxies as elliptical, spiral and irregular. Elliptical galaxies are spheroidal in shape, and range from perfectly round to quite flattened. They are designated by the letter E, followed by a number which describes the degree of flattening. Spirals, on the other hand, are thin disks with a central bulge and arms that wind out from the center. Spirals are designated by the letter S, followed by other letters that describe the size and strength of the spiral arms, as well as the relative size of the nucleus. Irregulars have no trace of circular or rotational symmetry but have an irregular or chaotic appearance. Irregulars are denoted by the letters Irr.
Elliptical galaxies, which essentially consist of only a nuclear bulge component are subdivided among seven ellipticity classes from E0 (circular) to E7 (cigar shaped). Numerically the ellipticity is given by 10(a-b)/a, where a is the length of the major axis and b is the length of the minor axis. Of course, the Hubble Classification does not tell us the true shape of the galaxy (e.g. an E0 could be a "cigar" seen down its barrell). Statistical arguments suggest that the distribution of galaxies among the ellipticities is roughly uniform. Spiral Galaxies Flattened systems which have a thin disk Display spiral structure. Divided into barred (SB) and unbarred (S) spirals. Further subdivided into classes a, b, and c; e.g. SBb, Sc, ... where a = large nuclear bulge & tightly wound spiral arms, c = small nuclear bulge & loosely wound spiral arms.
Lenticulars are similar to spiral galaxies in shape and color but no spiral arms. There are flattened systems which are morphologically between ellipticals and spirals. Irregular galaxies come in two types - Irr I which are in some sense a logical extension of the Hubble tuning fork, having characteristics "beyond" those of class Sc - high gas content, dominant presence of a young population. Irr I galaxies may show bar-like structures and incipient spiral structure like the Large Magellanic Cloud. Such galaxies are sometimes referred to as "Magellanic Irregular" galaxies. Irr II which are galaxies which defy classification because of some form of disturbance. For example, M82 is undergoing an intense period of star-formation.

Galaxy Characteristics are Uniquely Related to Classification
	E0-E7	S0	Sa	Sb	Sc	Irr
Nuclear Bulge	"All Bulge" No disk	Bulge &Disk	Large	~	Small	None
Spiral Arms	None	None	Tight/Smooth	~	Open/Clumpy	Occasional traces
Gas	Almost none	Almost none	~1%	2-5%	5-10%	10-50%
Young Stars HII Regions	None	None	Traces	~	Lots	Dominates Appearance
Stars	All Old (~ 1010yr)	Old	Some young	~	~	Mostly(?) young (but some v. old)
Spectral Type	G-K	G-K	G-K	F-K	A-F	A-F
Color	Red	Red	~	~	~	Blue
Mass (Msun)	108-1013	(More)  1012-109    (Less)				108-1011
Luminosity (Lsun)	106-1011	(More)  1011-108    (Less)				108-1011

How Do Galaxies Evolve?

The phenomenon of lookback time allows us to actually observe the evolution of galaxies. We are not seeing the exact same galaxies as today, but it is possible to trace the behavior of galaxies types with distance/time.

1. Galaxies could form (like stars) from the collapse of a gas cloud under gravity Globular clusters form on elliptical orbits as the gas falls towards the centre If the cloud has some net rotation, frictional energy loss in the gas will cause it to settle into a disc around a central bulge stars formed after this happens are already in circular orbits: disc stars stars formed before this do not settle into disc: halo dark matter also unlikely to settle into disc This model seems a plausible explanation for spiral galaxies.

2. Elliptical galaxies may form as a consequence of collisions and mergers They are more common in dense environments where collisions would be more common Their stars orbit in a less ordered fashion (result of disruption?) They sometimes show faint "shells" of higher star numbers (relics of shock waves from interactions?) They have little remaining gas (used up or ejected from galaxy?)

3. Colliding or otherwise interacting galaxies are observed Collisions initiate bursts of new star formation Stars and gas are lost in "tidal tails" Well reproduced by computer simulations

4. Spiral structure in spiral galaxies needs explanation If they were simply accidental concentrations of stars, they would "wind up" in a few orbits Presence of new stars suggests density wave of higher density gas (cf. sound wave) increase in gas density triggers star formation presence of more mass attracts nearby stars, affecting their orbits Such a wave could be generated by bar (for barred spirals), satellite galaxy or near-collision

5. We can test theories of galaxy evolution by studying very distant galaxies Light takes a long time to reach us => see cluster as it was a long time ago Pictures of very distant galaxies (e.g. Hubble Deep Fields) show higher proportion of interacting galaxies greater proportion of irregular or disrupted galaxies - fewer "normal" spirals large population of small, faint blue galaxies (which may later collide and merge to form modern galaxies)

6. Some distant galaxies show high levels of activity emanating from their central nuclei Such active galactic nuclei (AGN) much more common at great distances this is a temporary stage in the life cycle of young galaxies Observations indicate very high luminosity and presence of fast-moving hot gas Likely power source is accretion of gas by supermassive black hole Activity would cease if gas in central region depleted (material further out is in "safe" stable orbits: Sun in no danger of being swallowed by Milky Way's black hole) Evidence from nearby galaxies is that supermassive black holes are common (usual?) in centres of large galaxies: black hole mass larger for larger central bulge