Digital Universe Guide - Hayden Planetarium

42 3. THE MILKY WAY ATLAS
rings will grow as you increase the luminosity scale factor. If you find the labels are fading from view or
prefer a stronger color to the blue, you may change the label color by using the textcment command.
Change the labels to a gray color by using the command
textcment 1 .3 .3 .3
This changes color index 1 to a red-green-blue color of 0.3.
Next we’ll see how many of these exoplanets have heard from us when we discuss the extent of
Earth’s radio signals in space.
Tutorial: Earth’s Radio Sphere
Goals: Understand the relationship between light-travel time and distance in the Universe;
see the extent of Earth’s radio signals in the Galaxy.
Before starting, turn on: stars, mwVis, expl
You will be using: radec, 1ly
With the stars (stars group) and exoplanets (expl group) on, let’s turn on a marker showing where
Earth’s earliest radio transmissions have reached in space. Turn on the equatorial coordinates (radec).
From this perspective, this group no longer acts like a coordinate system but rather doubles as Earth’s
radio sphere.
The radio sphere shows the boundary of Earth’s radio signals beginning in the late 1930s and into
the early 1940s, when radar, television carrier waves, and atomic testing started sending strong radio
signals into space. Strong enough to pass through Earth’s ionosphere, these signals travel into
interstellar space at the speed of light. A signal that left in 1940 is, as of 2012, 72 light-years away. The
radio sphere then contains a history of Earth’s signals and implies what you likely know: looking out into
space means you’re looking back in time. At 57 light-years away, we would detect the broadcast debut
of The Honeymooners, and at 48 light-years, we would her the Beatles appearance on Ed Sullivan.
With the radio sphere and the exoplanets on, it is now possible to see how many planetary systems
have heard from us. How many do you count?
Fly away from the radio sphere so that you see most of the stars. The radio sphere is the farthest
extent of mankind’s presence in our Galaxy and the Universe, just 72 light-years from Earth. Turn on the
1,000-light-year grid, 1kly, and you begin to see just how small an influence we’ve had in the Galaxy. In
the coming tutorials, we’ll explore how much larger the Galaxy is, but let’s start with the stars.