Galaxy Season (and why is the night dark?)

M86 and Galaxies

M86 and galaxies, 4/11-12/2021 (click image for technical info).

Spring is galaxy season. The Milky Way winds low around the horizon leaving the thin part of the galaxy overhead making the best time to look up and out through our galaxy to other galaxies millions of light years away. The larger galaxies in this image range from 15 million to 40 million light years away. Our galaxy is estimated to be between 150,000 to 200,000 light-years in diameter making these galaxies well beyond the stars and objects inside the neighborhood of our Milky Way.

The two brightest, fuzzy objects in the right center of the screen are the elliptical galaxies M86 and M84. The two galaxies in the upper left are known as “The Eyes.”

Speaking of eyes, in 1823 Wilhelm Olbers used his to look up at night and wondered why it is dark at all.

Heinrich Wilhelm Matthias Olbers.jpg
Nice eyes. Heinrich Wilhelm Matthias Olbers (lithography by Rudolf Suhrlandt) – Wikipedia.

This time of year we expect the night sky to be darker without the Milky Way overhead and fewer stars to see. But Olbers wondered. If the universe were infinite and stars in the universe are distributed uniformly, he figured the night sky should be all light. Stars get dimmer the further they are away but the number of stars we see looking out through a given wedge of space increases with distance. It turns out, in one of those easy but elegant mathematical ways, the number of stars increases with distance in exactly the inverse ratio at which the intensity of starlight diminishes. The two effects, thought Olbers, should only cancel each other out. Stars get dimmer the further out you look, but there are equally more of them and every where we look we should see light.

But it is clearly not so in early April. The sky overhead is relatively devoid of stars and it takes a telescope to bring out more images. But even the image above, taken through my telescope, has plenty of dark space.

Why is the night dark?

April sky from Torrey, Utah. M86 is in the box above Virgo’s shoulder – Stellarium.

-Mark Bailey, Torrey, Utah, April 20, 2021

4 thoughts on “Galaxy Season (and why is the night dark?)

  1. KB

    So interesting and accessible!! I am so curious now…why IS it dark??!! You’re quite the storyteller dad. I can’t wait to build WilderHouse so you can bring your starry stories to my visitors 😌🥰

    Liked by 2 people

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  2. Scott Abbott

    Damn! Learning something new every morning. Thanks Mark

    From: Thots and Shots
    Date: Tuesday, April 20, 2021 at 9:45 AM
    To: Scott Abbott
    Subject: [New post] Galaxy Season (and why is the night dark?)
    Mark Bailey posted: ” M86 and Galaxies M86 and galaxies, 4/11-12/2021 (click image for technical info). Spring is galaxy season. The Milky Way winds low around the horizons leaving the thin part of the galaxy overhead and therefore a prime time to look up and out throug”

    Liked by 1 person

    Reply
  3. Ggreybeard

    It is worth pondering Olber’s paradox, because it is a common sense prediction which fails the test – and becomes a cornerstone of the Big Bang Theory.

    One wonders if the Universe really is infinite and I guess we will not answer that in our lifetime, if ever.

    Liked by 1 person

    Reply
  4. Mark Bailey Post author

    The paradox only seems to suggest more what the universe is not rather than what it is. Ha, I see the smoke start to curl out of your ears.

    Meanwhile, I hope your recovery is going well, Roger!

    Liked by 1 person

    Reply

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