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Polaris Aa
Details
Polaris Aa is the yellow supergiant at the heart of the North Star system and the nearest classical Cepheid to Earth. Because Cepheids act as standard candles for measuring cosmic distances, this pulsating star is far more than a navigation marker: it is one of astronomy’s key calibration beacons.
Polaris Aa is the heart of the North Star system, and that already makes it familiar. But the star itself is more interesting than the navigation role it plays. It is a yellow supergiant and a classical Cepheid, which means it is not static at all: it pulses in a regular rhythm.
That pulse matters because Cepheids are useful to astronomers as standard candles. Their changing brightness helps turn them into calibration tools for measuring cosmic distances. So Polaris Aa is both a reference point for direction and a reference point for scale, which gives it a rare double life in astronomy.
That is why Polaris Aa stands out. It is the star people imagine as fixed, yet it is one of the most scientifically useful examples of stellar change. Polaris Aa is a reminder that even the sky’s best-known marker is still a living star with its own rhythm.
23.9million km
Visual creditscaleofspace.org / CC BY-NC 4.0
Key facts
- Category
- Stars
- Object class
- Yellow supergiant Cepheid star
- Mass
- ~5.1 solar masses
- Host
- Polaris system
- Scale fact
- 64.4 million kmdiameter
- Estimated age
- ~45–67 million years
- Composition
- Hydrogen and helium
- Temperature
- ~6,015 K surface
Scale context
Where Polaris Aa sits on the full axis
Shared physical scale
217million km
Together, these objects make the size change around Polaris Aa easy to compare at a glance.
Between the smallest and the largest, perspective is everything.
About
Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides turn parts of that scale into curated essays, while focused views let you explore the same range through specific groups of objects.