Recreating Edwin Hubble's discovery of a Variable Star in the Andromeda Galaxy

[oopfan]

On October 6, 1923 renowned astronomer Edwin Hubble discovered a pulsating star in the Andromeda Galaxy which quickly led to the revolutionary discovery that M31 is a galaxy unto itself 2.5 million light-years away, and not a gaseous cloud of stars within our own Milky Way.

I dipped into my archive of astro images to see if I might have captured that same variable star. I did! In the attached image I've overlaid a small region of a 6-panel mosaic I made last year, atop Hubble's photographic plate he captured with the 100-inch telescope at Mount Wilson Observatory. You can see that the pattern of stars matches nicely. At the point of the arrow that I marked "VAR!" you will see an equilateral triangle of faint stars. You can see that it matches up with Hubble's. That one star at the vertex of the triangle is the famous variable star.

My image is a stack of 21x 90-second luminance frames captured with a cooled Atik 314E CCD and William Optics 71mm f/5.9 refractor under Bortle 5 skies. Total integration time is 31.5 minutes. I know nothing of the period of this variable star. Being a Cepheid type variable star its period is most likely one or more days in length, so integrating for 31.5 minutes is not a problem. The bigger question is, am I seeing it at minimum light or maximum light? I almost don't want to know. I want to feel the same sense of discovery as Hubble did.

EDIT: I suspect that this variable star is likely to have a long period perhaps months long. I suspect this because of the "period-luminosity relationship" which says that highly luminous Cepheids have long periods. This star must be highly luminous if I can see it as a point source from such a great distance. If it was less luminous it would be lost in the glow of the galactic arms.

I will make this one of my top priority projects this Fall: measure the period of the variable star, and then calculate its distance using Leavitt's Law.

https://apod.nasa.gov/apod/ap200426.html
https://apod.nasa.gov/apod/ap950701.html
https://astrotuna.com/andromeda-galaxy- ... 019-08-12/

Brian

[admin]

Hi Brian,

Another fascinating post – thank you for sharing these on the forums. It's amazing to think that we can now capture the same information with a 79 mm diameter scope and modern cameras that 100 years ago took the 100 inch telescope to capture. I wonder how long Edwin Hubble's exposure was?

Cheers, Robin

[oopfan]

Hi Robin,

Yes, I wondered the same. I searched and found an answer:

https://www.nasa.gov/mission_pages/hubb ... ar-v1.html

There is lots to read but allow me to pull a quote:

The astronomer [Hubble] spent several months in 1923 scanning Andromeda with the 100-inch Hooker telescope, the most powerful telescope of that era, at Mount Wilson Observatory in California. Even with the sharp-eyed telescope, Andromeda was a monstrous target, about 5 feet long at the telescope's focal plane. He therefore took many exposures covering dozens of photographic glass plates to capture the whole nebula.

He concentrated on three regions. One of them was deep inside a spiral arm. On the night of Oct. 5, 1923, Hubble began an observing run that lasted until the early hours of Oct. 6. Under poor viewing conditions, the astronomer made a 45-minute exposure that yielded three suspected novae, a class of exploding star. He wrote the letter "N," for nova, next to each of the three objects.

Later, however, Hubble made a startling discovery when he compared the Oct. 5-6 plate with previous exposures of the novae. One of the so-called novae dimmed and brightened over a much shorter time period than seen in a typical nova.

Hubble obtained enough observations of V1 to plot its light curve, determining a period of 31.4 days, indicating the object was a Cepheid variable. The period yielded the star's intrinsic brightness, which Hubble then used to calculate its distance. The star turned out to be 1 million light-years from Earth, more than three times Shapley's calculated diameter of the Milky Way.

I don't know how advanced photographic plates were in the 1920's but I suspect that it wasn't great by today's standards therefore I am not surprised reading about 45-minute exposures.

I got my start in astronomy in the early 1970's. CCD's were available back then but only to researchers with deep pockets. The amateur community used photographic emulsion. While Kodak Tri-X was rated at a fast 400 ASA it also suffered from something called Reciprocity Failure which caused the speed of the emulsion to decline rapidly after its initial exposure to light. For example, 400 ASA for the first second, then 100 ASA for next second, etc.

There were special emulsions available like Spectroscopic 103aE that was discussed in Sky & Telescope magazine. I got my hands on a couple rolls. It was significantly better but still suffered from Reciprocity Failure albeit at a lower rate. The longest single exposure I took with 103aE was 40-minutes of the Lagoon Nebula with my 2.4-inch f/11 refractor. It showed a good amount of nebulosity but the sky brightness quickly became a negative factor, mostly due to the southerly direction and New York City being 30 miles away.

There was word back then of something called "stacking" where people were taking shorter exposures on film and then "stacking" the developed film one atop the other. I never tried it due to my mount's inability to hold a star at a fixed position on the film. Registration was therefore a nightmare.

Brian

[SteveInNZ]

Much as I hate the term, Var1 is a bucket list object for me. That's not helped by me living in New Zealand, where M31 barely creeps up to 10 degrees azimuth.
I haven't seen it captured with anything less than 8" aperture before, so now I have renewed hope that I can get it with my travel setup. Thank you.

Steve.

[oopfan]

Hi Steve,

AAVSO's official designation is "M31 V0619". They report that its magnitude fluctuates between 18.5 and 19.8V. That is faint. I've known that my scope/cam can see down to 18th magnitude at my Bortle 5 site but can it see down to 19th magnitude? What makes things worse is that the star is embedded in the glow of a spiral arm which further reduces the signal-to-noise ratio. There is only one way to tell. Do it!

I was curious as to what phase the star was in when I captured my image. According to the AAVSO's Ephemeris it reached maximum light on July 31, 2019 07:08 UTC. My image was captured August 3, 2019 02:00 UTC. So, three days after maximum light. That pretty much puts it at magnitude 18.5.

Here is a link to a star chart:
https://www.aavso.org/apps/vsp/chart/?e ... type=chart

And here is the Phase Plot contributed by ten AAVSO members between 2010 and 2019. Ignore the outliers.

Brian