Callables allow you to define your own functions for calculating a few of the style properties for stars: size, opacity, color, and their labels. DSOs support callables for opacity and labels only. Starplot has a few basic callables built-in, but you can also create your own.

Here's an example that uses the built-in callable color_by_bv_gradient to create a horizon plot that colorizes each star with a radial gradient based on its B-V index — a bright core that fades out toward the edge, similar to how stars actually look to the eye:

from datetime import datetime
from zoneinfo import ZoneInfo

from starplot import Observer, HorizonPlot, styles, callables, DSO, _

tz = ZoneInfo("America/Los_Angeles")
dt = datetime(2025, 11, 18, 21, 30, 0, tzinfo=tz)

observer = Observer(
    dt=dt,
    lat=32.97,
    lon=-117.038611,
)

style = styles.PlotStyle().extend(
    styles.extensions.GRAYSCALE_DARK,
    styles.extensions.HORIZON,
)
style.axes.background.fill = {
    "stops": styles.gradients.NIGHT,
    "type": "linear",
}
style.figure.background.fill = "#000"

p = HorizonPlot(
    altitude=(0, 70),
    azimuth=(30, 150),
    observer=observer,
    style=style,
    resolution=2000,
    autoscale=True,
)
p.ground(min_altitude=5, max_altitude=8)
p.stars(
    where=[_.magnitude < 3.6],
    where_labels=[_.magnitude < 1.6],
    # callable to color the stars based on their BV index:
    color_fn=callables.color_by_bv_gradient,
    # callable to make dimmer stars semi-transparent:
    opacity_fn=lambda s: 1 if s.magnitude < 2 else 0.5,
)

style.gridlines.label.font_size *= 1.2  # make the labels 20% bigger
style.gridlines.label.fill = "#e5ecf3"
style.gridlines.line.width = 0  # hide the gridlines, we just want the labels
p.gridlines(alt_label_locations=[])

# Plot a dotted circle around the Pleiades (M45) and an arrow pointing to it
m45 = DSO.get(m="45")
p.polygon(
    geometry=m45.geometry,
    style={
        "fill": None,
        "stroke": "#fffb0e",
        "stroke_width": 6,
        "dash_array": "dotted",
    },
)
p.arrow(target=(m45.ra, m45.dec))

p.export("tutorial_08.svg")

For more details on using callables and an example of how to create your own, check out the reference page.

In the next section, we'll learn where to go from here...

8 - Using Callables