Hubble Essay

8/11/2019 Hubble Essay

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PHYS390: Hubble essay James Love (2955052)

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The observations in Hubbles 1929 paper, A relation between distance and radial velocity

among extra-galactic nebulae were performed to clarify phenomena previously examined

and were limited by the instrumentation of that era. The uncertainty of the data and the

inability to discern the distances of dimmer nebulae led Hubble to resist making any definite

conclusions about his observations. The results obtained can be viewed in the present as

laying the foundation for the discovery of the expansion of the universe.

Hubbles paper seeks an explanation to the involvement of a variable K term (of several

hundred kilometres) in calculating the suns motion with respect to extragalactic nebulae.He

states that explanations of this paradox have been sought in a correlation between apparent

radial velocities and distances and that the question should be re-examined using only

nebular distances that are believed to be reliable (1929, p168).

Observing the apparent brightness or luminosity of novae and blue stars involved in emission

nebulosity only gives the order of distances to extra-galactic nebulae; however, known

Cepheid variables provide a reference for obtaining numerical values (Hubble 1929, p168).

Once a suitable zero point of the period-luminosity relation has been determined, the distance

of Cepheid variables can be obtained from measurement of the period of their shifts in

apparent brightness or luminosity (Lang 2006, p37). Hubble states that this method is

restricted to the few nebulae which are well resolved by existing instruments (1929, p168).

The Mount Wilson observatory where Hubble performed his measurements had a diameter of

100, which corresponds to a diffraction-limited angular resolution of about 0.05 arc seconds

for visible wavelengths. The modern telescopes at Mount Wilson, in the CHARA (Centre for

High Angular Resolution Astronomy) array, have a combined resolving power of 330m,


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which corresponds to a diffraction-limited angular resolution 0.0003 arc seconds

(Heckert 2009). Hence, with greater light gathering power and the ability to resolve images of

more closely-spaced Cepheid variables and the nebulae themselves, modern observations in

the visible wavelengths would allow more reliable distance measurements to be obtained

using absolute luminosities and the period-luminosity method (Freedman & Kaufmann 2008,

p139). Also, modern CCD technology captures 70% of the visible light falling onto it, as

opposed to the 2% for photographic film, so dimmer, more distant objects can be detected

(Freedman & Kaufmann 2008, p142).

Modern multi-waveband observations allow a greater range of wavelengths to be detected

and hence, larger and more numerous redshifts. This would allow the velocities of more

distant nebulae to be measured accurately, resulting in a more substantial distribution to aid

the determination of the K term (Freedman & Kaufmann 2008, p647). This is particularly

relevant to the requirement of more data on the peculiar motion of the distant Virgo nebulae

(Hubble 1929, p170).

The data deemed reliable in 1929 displayed a linear relationship between distances and

velocities, which suggested a new solution where distances were introduced as coefficients of

the K term, i.e. , so that K represented the

velocity at unit distance due to this effect. The probable order of the values for these

coefficients were taken as round numbers between results for 24 nebulae individually and the

other combining them into 9 groups based on proximity of direction and distance. Strmberg

checked the order of the values, A = 277, D = +36, V0= 280km/s, K = +500km/s/Mpc by

independent solutions for the different data groupings (Hubble 1929, p170-1).


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The results were displayed in a graphical form, where radial velocities corrected for solar

motion were plotted against distances estimated from involved stars and mean luminosities

of nebulae in a cluster. Distances for the 22 nebulae whose values were at the time

unaccounted for, were calculated from velocities corrected for solar motion, and absolute

magnitudes were derived from apparent magnitudes (Hubble 1929, p172).

The results obtained show a roughly linear relationship between velocities and distances;

however, the need to examine more distant nebulae prevents Hubble from reaching any

definite conclusions in the paper. He outlines the possibility that that solar motion with

respect to clusters represents the rotation of the galactic system, and also the possibility that

the data implies something about the curvature of space, or the de Sitter effect (1929, p173).

In hindsight, it is clear that this paper lays the foundation for the discovery that the universe is

expanding.


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References:

Freedman, R & Kaufmann, W 2008, Universe, 8thEdition, W. H. Freeman and Company, New York.

Heckert, P 2009,History of Mount Wilson Observatory, accessed 28/10/2009,http://astronomyhistory.suite101.com/article.cfm/history_of_mount_wilson_observatory

Hubble, E 1929, A relation between distance and radial velocity among extra-galactic nebulae,PNAS, vol. 15.

Lang, K 2006,Astrophysical Formulae, 3rd

edition, Birkhuser, Switzerland.

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Hubble Essay