By Stacy Palen

Gaia’s latest data haul, from December 2020, includes the proper motions of more than 1 billion stars. So the Gaia astronomers did the fun thing and mapped their future positions as they move against the background of the Milky Way.

Below are some questions to ask your students based on this article.

1). There are two motions that are discussed in this article. One of them, “proper motion,” is the “nearly imperceptible motions across the Galaxy year after year.” The other is parallax. How could astronomers tell these two motions apart?

Answer: Parallax is a back-and-forth motion. The star returns to its starting point after a year. The proper motion adds every year, with the star moving farther and farther from its starting point.

2). Why is it important to know the distance to stars?

Answer: Because the distance measurement is connected to the luminosity measurement, and the luminosity is necessary to find out about stars' size, age, structure, and evolution.

3). Study the image in the article that shows the star trails. Are there any trends in the motions of the stars shown here?

Answer: Yes. As you look toward the galactic center, they seem more random, whereas the farther out, toward the corners of the image, the trails are more parallel to one another.

4). How many years will it take each of these stars to travel along one of those trails?

Answer: 400,000 years.

5). Do you expect that you will see any of these stars move a significant distance in your lifetime?

Answer: Absolutely not.

6). Working backwards from the trails that you see here, have the visible constellations changed significantly since the pyramids were built…a bit more than 4,000 years ago?

Answer: No. The stars will have moved about 1/100th of the way along a track in that time, which is not very far.

By Stacy Palen

In 2019, astronomers released the first image of the shadow of a black hole’s event horizon. Further processing of earlier data shows that the accretion disk has rotated.

Below are some questions to ask your students based on this article.

1). What is the black circle in the middle of the image?

Answer: This is the shadow of the event horizon. It is larger than the event horizon and shows where the light has been bent away from the line of sight and into the black hole.

2). What is the bright ring around the outside?

Answer: This is light coming from the accretion disk around the black hole. This light has been bent by the black hole’s gravity into our line of sight.

3). A series of models of the black hole are shown in the figure titled "M87* black hole appearance in 2009-2017." If the bright spot were on a clock face in 2009, it would have been at about 5:00 p.m. Where was it in 2017?

Answer: Nearly all the way around to 5:00 p.m. again, so about 6:00 p.m.

4). If this motion is periodic (that is, it repeats over and over, taking the same amount of time each time), what is the period?

Answer: 8 years.

5). How long will astronomers have to wait to find out if this motion is periodic?

Answer: At least one more period, so 8-16 years.

6). In the video and written text, the black hole is described as "sucking up light." Many astronomers object to this terminology, insisting that black holes don’t "suck in light" but that "light falls in." What distinction are astronomers trying to make?

Answer: Black holes have gravity rather than some super-strong supernatural force.