M31-V1, the star Hubble viewed and changed our universe

If I were to leave a blank after the word "Andromeda", then chances are that you would be filling it in with the word "galaxy". This is because the Andromeda galaxy is the nearest major galaxy next to our own Milky Way, making the association natural. A little over 100 years ago, however, nobody would have filled it with the word "galaxy".

That's because even early in the 1920s there was a raging ongoing debate about such "spiral nebulae," Andromeda being one of many such faint, fuzzy patches of light. On one side were those who believed that the Milky Way was the entire galaxy and that these nebulae were just smaller objects within, and on the other side were those who believed they were galaxies in their own right, extremely large and at rather vast distances.

The discovery of V1 by American astronomer Edwin Hubble helped settle that debate once and for all. What's more, it turned out to be a crucial first step in enabling Hubble to unlock a grander, larger universe.

Searches with Hooker

The 100 inch Hooker Telescope at the Mount Wilson Observatory. | Photo Credit: Ken Spencer / Wikimedia Commons

Back in 1923, Hubble spent several months scanning what we now know as the Andromeda galaxy, also known as Messier 31, or M31. He had at his disposal the 100-inch Hooker telescope at Mount Wilson Observatory, the most powerful telescope of the time.

That, however, didn't make his objective simple, as Andromeda was a colossal target. It spanned 5 feet long along the telescope's focal plane, forcing Hubble to take plenty of exposures covering dozens of photographic glass plates in order to cover the whole of it.

The viewing conditions on October 5, 1923 were far from ideal as Hubble focussed on one of the three regions he usually concentrated on. During an observing run that went into the early hours of October 6, Hubble made a 45-minute exposure that revealed three suspected novae, a class of exploding star.

'N' turns 'VAR!'

Against each of these, Hubble wrote the letter 'N' -- for nova -- on the plate identified as H335H (Hooker plate 335 by Hubble). To his surprise, one of the three actually appeared on earlier plates, including H331H that he had taken during the previous night's observation on October 4.

Rummaging the archives, Hubble was able to discern that this one appeared as far back as 1909. As it dimmed and brightened far too quickly when compared to what is typical of a nova, Hubble got to work, plotting its light curve.

Henrietta Swan Leavitt. | Photo Credit: Margaret Harwood / Wikimedia Commons

What is a Cepheid variable?

Cepheid variables, also called Cepheids, are a type of variable star that brighten and dim periodically.

By the time Hubble discovered V1, astronomers were already aware that the period of time over which Cepheids varied depended on their intrinsic luminosity.

This was thanks in large to American astronomer Henrietta Swan Leavitt. While working for the Harvard College Observatory, Henrietta discovered the relationship between a Cepheid's period and true luminosity. She noticed that the longer Cepheids took to pulsate, the brighter, and hence likely larger, they were.

In a 1912 paper, Henrietta published a graph "showing that there is a simple relation between the brightness of the Cepheid variables and their periods." This period-luminosity relationship is sometimes referred to as the Leavitt Law.

"She deserved the Nobel Prize for her work," Hubble often said about Henrietta. There were a few who were willing to nominate her, but Henrietta prematurely died of cancer in 1921. As Hubble's discovery came after her death and as the Nobel Prizes are not awarded posthumously, she was never considered for the award.

With that graph, Hubble was certain that the object was in fact a Cepheid variable, a type of star that had already proven to be a reliable marker for distance within our galaxy. He took out his red marker, crossed out the "N" and wrote "VAR!" next to the newfound Cepheid variable.

The "VAR" obviously stands for "variable". In case you are wondering why Hubble chose to have an exclamation along with it, it was because he realised the magnitude of what he had just discovered.

Settles the debate

By determining its period from the light curve, Hubble was able to arrive at the star's intrinsic brightness. This then enabled him to calculate its distance, something that astronomers needed to settle their ongoing debate. He arrived at a figure of one million light-years, multiple times more than the largest estimate for the diameter of the Milky Way. M31-V1 was undeniably outside the Milky Way.

Hubble continued to gaze at Andromeda, seeking more data to substantiate his efforts. By the time 1924 came to a close, he had identified 36 variable stars in Andromeda, and one-third of it -- 12 -- were Cepheids.

Based on all these Cepheids, Hubble arrived at a distance of 900,000 light-years separating Andromeda and the Milky Way. The errors that had forced the astronomers of the 20ᵗʰ Century to overestimate the diameter of the Milky Way had crept in to force Hubble to underestimate the distance between the two galaxies. Current estimates place the Andromeda galaxy at a distance of 2.5 million light-years from the Milky Way.

"The history of astronomy is a history of receding horizons"Edwin Hubble

Plenty to understand

Hubble's discovery of M31-V1, in fact, turned out to be just his first step towards uncovering more secrets of the universe. In the years that followed, Hubble found even more galaxies beyond the Milky Way and these enabled him to arrive at the concept of an expanding universe.

"The history of astronomy is a history of receding horizons," Hubble had stated when describing his principal observations and conclusions in the Silliman Memorial Lectures at Yale University in 1935, later compiled and published as The Realm of the Nebulae in the following year. Hubble certainly did his bit to expand the boundaries of what we know, while at the same time acknowledging that plenty still remains to be understood.

M31 Cepheid variable star V1 as viewed by the Hubble Space Telescope in 2010-11. | Photo Credit: NASA, ESA, Hubble Heritage Project; Acknowledgment: Robert Gendler

When Hubble viewed V1 again

Nearly 90 years after Hubble's breakthrough discovery, Hubble was at it again, viewing V1. In case you are wondering how someone who died in 1953 was viewing a Cepheid star in the 21ˢᵗ Century, then be assured that we are talking about his namesake, the Hubble Space Telescope.

By directing the telescope at V1, NASA's astronomers were not only paying homage to the revered astronomer's milestone observation, but also reminding the world at large, and astronomers in particular, that Cepheids remain relevant.

For six months in 2010, observers -- including amateur astronomers -- followed V1, producing a light curve of the rise and fall of the star's light. Based on this data, the Hubble Space Telescope was targeted at V1 in 2010-11 to capture it at its brightest and dimmest phases.