Night Notes

A field guide to the night sky

Look up.

What you are seeing, when to look, and how to see more of it. Short notes for anyone with a clear night and ten spare minutes.

Star colors

Every star has a temperature, and you can see it.

A star glows like hot metal. Cooler stars shine orange and red, hotter ones white and blue. Astronomers sort them into seven classes, from the hottest to the coolest: O, B, A, F, G, K, M.

  1. Oabove 30,000 KZeta Puppis
  2. B10,000–30,000 KRigel, Spica
  3. A7,500–10,000 KSirius, Vega
  4. F6,000–7,500 KProcyon
  5. G5,200–6,000 Kthe Sun
  6. K3,700–5,200 KArcturus, Aldebaran
  7. M2,400–3,700 KBetelgeuse, Antares

Try it on a winter evening: Betelgeuse and Rigel sit at opposite corners of Orion. One is clearly orange, the other blue-white. Their surfaces differ by more than 8,000 degrees.

Finding north

Two stars point the way to Polaris.

Find the bowl of the Big Dipper. The two stars on its outer edge, Merak and Dubhe, are called the Pointers. Follow the line from Merak through Dubhe about five times their distance, and you reach Polaris.

Polaris is not the brightest star in the sky; it ranks around 48th. It matters because it barely moves. The whole sky turns around it, and its height above the horizon equals your latitude.

At 45° N, Polaris stands 45° above the northern horizon. Everything inside the dashed circle never sets.

    The northern sky on a November evening. The sky turns counterclockwise around Polaris once a day.

    Light travel time

    You always see the sky as it was.

    Light covers about 300,000 km every second, which is fast but not instant. The farther the object, the older the light reaching your eyes tonight.

    1.3 sthe Moon
    8.3 minthe Sun
    8.6 yrSirius
    ~430 yrPolaris, estimates vary
    2.5 M yrAndromeda Galaxy

    The life of a star

    Mass decides how a star lives and how it ends.

    Every star forms in a cold cloud of gas and dust and spends most of its life fusing hydrogen. What comes next depends almost entirely on how heavy it is.

    Stars like the Sun

    1. NebulaGas and dust collapse under gravity
    2. Main sequenceAbout 10 billion years of steady hydrogen fusion
    3. Red giantThe core runs out of hydrogen; the star swells
    4. Planetary nebulaOuter layers drift away as a glowing shell
    5. White dwarfAn Earth-sized ember that cools for billions of years

    Stars above about 8 Suns

    1. NebulaA larger, denser cloud collapses
    2. Main sequenceOnly a few million years: heavy stars burn fast
    3. Red supergiantFusion climbs through heavier elements up to iron
    4. SupernovaThe core collapses; for weeks the star can outshine its galaxy
    5. Neutron star or black holeWhat remains depends on the mass of the core

    Meteor showers

    The same dates every year.

    Each shower happens when Earth crosses the dust trail of a comet or asteroid. The rate is the zenithal hourly rate (ZHR): meteors per hour under a perfectly dark sky with the radiant overhead. Real counts are usually lower.

    ShowerPeakZHRComes from
    QuadrantidsJan 380asteroid 2003 EH1
    LyridsApr 2218comet Thatcher
    Eta AquariidsMay 650comet Halley
    PerseidsAug 12100comet Swift–Tuttle
    OrionidsOct 2120comet Halley
    LeonidsNov 1715comet Tempel–Tuttle
    GeminidsDec 14150asteroid 3200 Phaethon
    UrsidsDec 2210comet Tuttle

    Best time is usually after midnight, when your side of Earth faces forward into the dust. A bright Moon near the peak hides the faint ones.

    Darker skies

    How dark is your sky?

    The Bortle scale rates the night sky from 1, a truly dark site, to 9, a city center. Next to each class is the faintest star magnitude you can expect to see with the naked eye. Higher numbers mean fainter stars.

    1. 1Excellent dark site7.6–8.0
    2. 2Truly dark site7.1–7.5
    3. 3Rural sky6.6–7.0
    4. 4Rural to suburban6.1–6.5
    5. 5Suburban sky5.6–6.0
    6. 6Bright suburban5.1–5.5
    7. 7Suburban to urban4.6–5.0
    8. 8City sky4.1–4.5
    9. 9Inner city4.0 at best

    Give it twenty minutes

    Your eyes keep adapting to the dark for 20 to 30 minutes. One look at a phone screen resets most of it.

    Use red light

    Dim red light barely affects night vision. That is what the switch at the top of this page is for.

    Look a little to the side

    The edge of your vision is more sensitive to faint light than the center. Faint stars and galaxies appear when you stop looking straight at them.