July 23 - Happy Birthday, Vera Rubin

Posted on July 23, 2020

Today's famous birthday was an American astronomer who was born in Philadelphia (Pennsylvania) in 1928.

Vera Rubin studied galaxy rotation rates - and she discovered a puzzle: the predicted angular motion of each galaxy didn't match the motion she observed.

If you think about how tiny galaxies appear in our skies...

This is the largest of the nearest galaxies -

and it's just a tiny smudge to the naked eye,

even under the darkest-sky conditions...

...then you may be wondering how Rubin measured any galaxy's rotation?

It's not like we're treated to sights like this!

Rubin and other astronomers take advantage of the Doppler effect. Light that is coming towards us is bluer than expected, and light that is going away from us is redder than expected. So an astronomer looks for galaxies with bluer light on one side and redder light on the other - and then she can compute the rotational speed.

Well, Rubin measured many, many galaxies' rotation - and she discovered that galaxies rotated quickly enough that they should fly apart! Since they didn't fly apart, she knew that there must be more mass - about five times more! - than we could see.

A Swiss astronomer named Fritz Zwicky had already proposed the existence of dark matter - matter we can't see, but that we can tell exists because of its gravitational pull. Rubin's work was some of the first evidence for his idea - but at first many scientists were very skeptical!

However, more and more scientists were able to confirm Rubin's data, find more evidence to back the theory that dark matter exists, and that there is WAY more of it than there is of matter we can see! 

Astrophysicists now believe that the normal matter
we see all around us and across our night skies is
only a very small part of what the universe if made of!

Also on this date:

Ethiopian ruler Haile Selassie's birthday 

Revolution Day in Egypt

Gorgeous Grandmas Day

Anniversary of the signing of a boundary treaty

Anniversary of a cloning success

Anniversary of the opening of the first swimming school in the U.S.

Birthday of Maori queen, Dame Te Atairangikaahu

Children's Day in Indonesia

Anniversary of the launch of the first Earth-resources satellite

Anniversary of the discovery of a comet

Anniversary of the discovery of an Earthlike exoplanet 

Renaissance Day in Oman

Teej Festival

(Date varies)

Plan ahead:

Check out my Pinterest boards for:

• July holidays
• July birthdays
• Historical anniversaries in July

And here are my Pinterest boards for:

• August holidays
• August birthdays
• Historical anniversaries in August

August 5, 2011 - Quasar Breakthrough

– 1962

In the late 1950s, astronomers discovered radio sources in the sky that had no visible counterpart. By 1960 hundreds of these radio sources, which seemed starlike in size, had been detected—and as astronomers searched the skies for one that could be seen as well, one was finally spotted. It looked like a faint blue star, but the spectrum obtained had completely unknown emission lines. The quasi-stellar radio sources remained a big mystery.

Quasar is an acronym for “quasi-stellar.” The term was invented by Chinese-born American astrophysicist Hong-Yee Chiu.

It was on this day in 1962 that another faint “star” was identified as the quasar 3C 273. This quasar's spectrum was also very odd—and scientists realized that it was really just a normal spectrum red-shifted so far that it was hard to recognize.

Wow—what does all that mean?

Well, the electromagnetic spectrum is like a rainbow of energy that is emitted from something. Your body and planet Earth emit infrared radiation—heat--and much hotter objects such as stars emit more energetic radiation, including visible light. A spectrum from a star such as our sun looks a rainbow because there are elements emitting various different colors of light.

Each element, when heated, emits its own characteristic spectrum, and the most common element in the universe, hydrogen, emits four specific, known spectral lines. What scientists realized as they studied quasars' spectra is that they were seeing familiar hydrogen spectral lines, with the expected spacing between the lines, but the lines were moved way over to the red end of the spectrum. That's what we call a red-shift.

A red-shift means the light source is moving away from us, the viewers. (And a blue-shift occurs when the light source is moving toward us.) This is similar to the way sound seems to change pitch depending on whether the source is moving toward or away from the listener. Have you ever heard a passing fire truck running its siren or train sounding its whistle? The sound dramatically drops from higher to lower pitch the moment the vehicle's sound source passes you. The faster the firetruck or train is going, the bigger the difference in the sound. And this sort of shift (also called the Doppler effect) happens in visible light (or other electromagnetic radiation) as stars or galaxies move toward or away from us.

Almost all galaxies are moving away from us. You've heard of the Big Bang, right? Well, the galaxies are all flying apart from one another, and we can tell the distance of a particular galaxy by seeing how much the light it produces is red-shifted—the farther away, the quicker it seems to be moving away, and the more red-shifted the spectral lines.

Scientists are now quite sure that the mysterious quasars are the very energetic centers of very, very distant galaxies. These compact regions surround the galaxies' supermassive black holes.

November 29, 2009

Happy Birthday, Christian Doppler

On this date in 1803, Christian Doppler was born in Salzburg, Austria. The son of a stone mason, Doppler became a mathematician and physicist. He is known for his explanation of the phenomenon called the Doppler effect.

When a train is approaching you and blowing its whistle, the whistle sounds higher-pitched than it does as it passes you and starts moving away. That's because of the Doppler effect.

Most of us have the opportunity to hear the Doppler effect in action when an emergency vehicle approaches and passes us—the siren seems higher as it nears us, then slides to a lower pitch as it passes and recedes.

Listen to the Doppler effect on a car horn here.

Why does the Doppler effect occur?

As an ambulance approaches us, emitting the siren at the same steady pitch (or frequency), the the ambulance's motion puts it closer to the sound wave that is traveling away from us. In other words, the sound waves bunch up in the direction that the ambulance is moving, and stretch out behind the ambulance. The bunched-up waves hit our ears more often, so the sound is higher. The stretched-out waves hit our ears at a lower frequency and sound lower.

Some animations on this site shows how it works.

And this animation actually shows an ambulance and should be easier for young children to understand.

Note that the sound being emitted doesn't change in pitch—it just SEEMS to change to you, the listener, standing still by the road. If you were IN the ambulance, the sound would be steady and unchanging. If both you and the ambulance were still, the siren would again sound like a steady pitch.

The Kettering University website also has (lower down on the webpage) an animation to explain a sonic boom. A sonic boom is made when a plane travels faster than sound—the plane passes us before the sound reaches us, and all the bunched-up sound waves sound like a single thump. (Well, okay, a double thump, one for the nose and one for the tail of the plane. Usually the two thumps are so close together, most people hear them as one.)

Here is another Doppler-effect animation—and this time, YOU control the direction and speed of the movement. Try moving it slowly at first to see the waves bunch up and stretch out. Then try moving it quickly to see what happens when a plane moves faster than sound.

And here is a YouTube video about sonic booms. Did you know that you can SEE as well as HEAR a plane break the sound barrier?

The Doppler Effect in Astronomy

Christian Doppler first explained the phenomenon of the Doppler effect, not for sound, but for light waves. He had been studying binary stars; as they circle each other, each star seems to approach us slightly and then recede again, rhythmically. We see the approaching and receding motions as shifts in the star's spectrum; as a star comes closer, its light is blue-shifted, and as it recedes from us, it's red-shifted.

The Doppler effect is one tool we use to find out about the structure, history, and future of the universe—including the facts that there was a Big Bang and that, billions of years from now, the Andromeda galaxy will collide with our Milky Way galaxy!