In the 1700s, it was clear that the Messier objects such as M-31 are not stars. They are also not comets, as they they don’t move through the sky. Messier actually cataloged these objects so he wouldn’t confuse them with comets, which also look like fuzzy patches in the sky.
Space Detective
A research team studied photographic plates from the Harvard College Observatory. From the plates they were able to gather about 500 historical data points. This allowed them to prove that two supermassive black holes are orbiting each other.
Ingredient X
X-ray astronomy presents an additional challenge because you not only have to put your x-ray telescope in space, you also have to build your telescope very differently.
Having FITS
For scientific imagery you want the data your camera gathers be “raw.” In other words, you don’t want the image to be compressed or manipulated in any way. For this reason a different image format is used, known as the Flexible Image Transport System, or FITS.
So What About Light?
From the photon’s perspective, the instant it is emitted in the Andromeda galaxy, it strikes your eye.
As the World Turns
It can be quite useful to have a stationary star in the sky, particularly when trying to navigate the open oceans without modern tools such as GPS. So Polaris is a happy coincidence. Of course, Polaris hasn’t always been the north star, nor will it be for long (on an astronomical scale).
Equation of Time
In our modern age, we measure time by clocks calibrated to an international standard. With the exception of the occasional leap second, each day is exactly 24 hours long. So you might figure that the time between noon on Monday and noon on Tuesday is likewise 24 hours, but things are not quite so simple.
Mutual Attraction
The first known binary stars were observed in the 1800s, but it wasn’t until the 1900s with the introduction of the filar micrometer that decently accurate measurements could be made. This device allowed you to center your telescope on the primary star, and then measure the position of the secondary star relative to the primary. By taking measurements over time (sometimes years or decades) you can start to see the companion star trace its path.
A Biased View
The figure below shows the positions of more than a thousand galaxies in the universe. You might think that tells us things about the history and evolution of the universe, and you’d be right. But it also tells us something about how we observe the universe. Knowing the latter is important, because all measurements have biases. If you don’t account for observation biases, you might mis-interpret your data.