# B Milky Way Galaxy in 1900

1. May 13, 2018

### Thecla

I read that in 1900 astronomers thought that our own Milky Way Galaxy was the only galaxy in the whole universe. This puzzles me because I presume they knew about the pinwheel structure of our galaxy and there were telescopes back in 1900( Yerkes for instance)which were large enough that could see the pinwheel structure of Messier objects and even more distant galaxies.

2. May 13, 2018

### mathman

In 1900 there were no standard candles to measure distant objects. Cepheid variable stars luminosity distance relationship was first discovered in 1908. As a result they didn't know how far away distant objects were.

https://en.wikipedia.org/wiki/Cepheid_variable

3. May 13, 2018

### Thecla

I know that the standard candle to measure distance, the Cepheid Variable, was discovered later. But seeing all these pinwheels in the sky should have given them a clue, provided they knew the Milky Way was a pinwheel. There are no pinwheel nebulae within our galaxy. They had to be external.

4. May 13, 2018

### Staff: Mentor

How do we know that?

5. May 13, 2018

### phyzguy

The spiral nebulae were thought to be solar systems in our galaxy in the process of forming. Nobody knew about the spiral structure of our own galaxy. We cannot see our own galaxy from the outside, so it is not obvious that it is a spiral structure. It was only by painstakingly mapping out the gas clouds in our galaxy using radio astronomy (which didn't exist in 1900) that we determined that the Milky Way has a spiral structure.

6. May 14, 2018

### stefan r

Hurricanes and typhoons have pinwheel cloud structures. Hurricanes are usually less than 1000km diameter and they are much closer than the moon. A lot of planetary nebula are inside of our galaxy.

Edit: also we have images of protoplanetary disks and sometimes they have spirals. Picture from alma, picture from hubble.

Last edited: May 14, 2018
7. May 14, 2018

### CygnusX-1

In 1900, no one knew the Milky Way was a spiral, and there was good evidence that the spiral nebulae lay inside the Milky Way.

Lord Rosse found the first celestial spiral in 1845; he then found many other spirals. "Rosse's discoveries renewed speculation that some of the nebulae were external galaxies. They also gave rise to the first suggestion, in 1852 by American astronomer Stephen Alexander, that the Milky Way itself was a spiral - a speculation that took ninety-nine years to confirm.

"During the latter half of the 1800s, however, in one of those curious backward steps that astronomy occasionally takes, several events conspired to undermine belief in external galaxies." - The Universe at Midnight by Ken Croswell, page 19.

Croswell goes on to list these as:

1. The 1864 discovery that the Orion Nebula is made of gas, which suggested that ALL nebulae, including the spirals, were gaseous objects similar to the Orion Nebula.
2. The 1869 discovery that the spiral nebulae lay away from the Milky Way's plane, suggesting they were part of our Galaxy.
3. The 1885 "nova" in the Andromeda Nebula. If the Andromeda Nebula lay outside our Galaxy, then that "nova" would have been impossibly powerful.

"These observations convinced astronomers that the spiral nebulae lay within the Milky Way. The question whether nebulae are external galaxies hardly any longer needs discussion,' wrote British author Agnes Clerke in 1890. It has been answered by the progress of discovery. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way.' In short, there was only one galaxy in the universe: our own." - The Universe at Midnight by Ken Croswell, page 21.

In the 1910s, Vesto Slipher's discovery of the high velocities of the spiral nebulae suggested they lay far beyond the Milky Way. The following decade, Edwin Hubble's discovery of Cepheids in nearby spirals demonstrated this beyond all doubt.

Finally, two corrections: Henrietta Leavitt discovered the period-luminosity relation for Cepheids in 1907, not 1908. And the discovery that the Milky Way is a spiral came in 1951 from optical observations, not radio observations. References to both assertions are available upon request.

8. May 15, 2018

### phyzguy

I'd like to see the reference that the spiral structure of the Milky Way was confirmed through optical observations. I think people had speculated that there might be a spiral structure, but I think the paper below by Oort, et.al., using radio observations of gas clouds, was the first convincing map:

Oort, J. H.; Kerr, F. J.; Westerhout, G., "The galactic system as a spiral nebula", Monthly Notices of the Royal Astronomical Society, Vol. 118, p.379, 1958.

9. May 15, 2018

### CygnusX-1

The discovery of the Milky Way's spiral arms is described in the seventh chapter of The Alchemy of the Heavens by Ken Croswell, who interviewed two of the three discoverers (William Morgan and Donald Osterbrock) and quotes them at length. In 1951, they mapped out ionized hydrogen regions to discover the spiral arms near the Sun. Croswell also cites the 1958 radio work you mention, but that came later.

Also see Owen Gingerich's article "The Discovery of the Spiral Arms of the Milky Way," which appears in The Milky Way Galaxy, IAU Symposium 106, page 59. As Gingerich writes, "I think there is some larger justice in the circumstance that the optical studies, on which so many decades of effort had been spent, narrowly won the race with the new and powerful radio astronomy to establish the fact that our galaxy really did have spiral arms, as had long been conjectured."